You shop online, you save money. Just about everybody believes that and comparison shops online. And these days, mortgage rates are easy to shop for online, too.

It’s convenient. Immediate. We’re programmed to believe it. But are online mortgage lenders consistently offering the lowest mortgage rates?

Manish Grover, a marketing consultant in the New York City area, certainly seems to believe so. He sent NerdWallet an email telling us just that.

“I’ve taken out two mortgages now from online lenders advertising on Zillow, and both times I’ve got a rate which is at least 0.50% lower than my local banks or my primary bank,” he says. “And I am considering the rate after all costs are considered (APR).”

Grover seems to know what he’s doing. Comparing annual percentage rates is definitely the way to go, because that’s an interest rate that considers all of the estimated fees and expenses that are built into a loan.

“In addition, the processing complexity is much lower as well,” Grover writes. “So my question is: What are some of the factors that help these online lenders to undercut banks? Is it because they don’t have physical infrastructure and do most business on phone and email?”

Good question. But before we determine how they do it, we’ve got to find out if online lenders really do offer the lowest rates on a day-in and day-out basis.

NerdWallet tracks the daily rates of national lenders for our Mortgage Rate Index. So we pulled three of the biggest banks from our daily survey to represent the brick-and-mortar constituency, as well as Quicken Loans, the largest online mortgage lender.

To even the field, we asked LoanDepot and Guaranteed Rate, arguably the two biggest runners-up in online home loan lending, to provide us their published rates for the same time frame. We heard back from Guaranteed Rate but not LoanDepot.

We considered all of these published rates for 30-year fixed-rate loans from June 16 through July 19, just before, during and after Britain voted to leave the European Union, sending mortgage rates falling dramatically. Here’s what we found:

Lenders price their mortgages based on a variety of parameters:

There are other layers to the process, but that’s the foundation.

Our mortgage rate comparison in the chart below comes with a big asterisk. Here, hold on to it and we’ll talk about that shortly.

In our initial analysis of four lenders (Bank of America, Chase, Quicken and Wells Fargo), Chase was clearly the low-rate winner (shown in yellow). Then we added in Guaranteed Rate.

One thing we noticed is that Guaranteed Rate doesn’t move its rates as much as the other lenders surveyed. Guaranteed Rate names a rate and sticks with it for days, sometimes weeks at a time. And for a handful of days, Guaranteed Rate really nailed the lowest rate. The rest of the time, it was pretty much neck-and-neck with Chase.

Meanwhile, Quicken, our other online contender, is basically at the top of the rate pack with Wells Fargo.

Here are the rate averages for the period for each lender (from lowest to highest):

Guaranteed Rate: 3.40%
Chase: 3.44%
Bank of America: 3.65%
Quicken: 3.69%
Wells Fargo: 3.70%

Shopping mortgage rates online has some serious drawbacks — and it’s all part of that big asterisk. You see, each lender has its own assumptions for the published rates it quotes. And the differences can be pretty significant.

Consider some of the assumptions made by each lender:

It’s all in the details: The location and value of the property, your credit score, your down payment and especially the discount points charged.

All of the lenders say that the rates quoted are for “informational purposes only,” “are subject to change without notice” and “your loan’s interest rate will depend” on a whole mess of stuff.

In other words, don’t count on this rate.

Online lenders may not necessarily offer the consistently lowest mortgage rates. Sorry, Manish. But to his credit, Manish shopped around and got the best rate he could find, online or otherwise. And that’s the real lesson.

It’s also important to note that in shopping for a rate online, you’re strolling through a big, dimly lit store. We compared a bunch of mortgage rates, all of which depend on varying factors. To really know what your rate is going to be, you have to submit an actual application — to several lenders. Because if there’s one thing apparent, it’s that every lender is pricing its loans a little differently.

But say you can save just 0.30% on a $300,000 home loan. The difference between scoring a 30-year mortgage at 3.40% rather than one at 3.70% saves you a little more than $50 a month — and over $18,000 in interest over the life of the loan. And shopping multiple lenders is likely to save you even more.

With thousands of dollars in play, over more than just a few years, small differences can add up to big money.

Hal Bundrick is a staff writer at NerdWallet, a personal finance website. Email: hal@nerdwallet.com. Twitter:@halmbundrick

The article Do Online Lenders Offer Lower Mortgage Rates? It Depends originally appeared on NerdWallet.