There are several ways you may have been given a refund. In all cases, be sure first to call your servicer and confirm what this is, how it was calculated, and get someones name in writing so you can go back to the source if the data is incorrect.
Here's what could have happened:
1) You buy for $1m. Your property taxes are calculated at 1.6% (Base, plus MR). The lender gets the 2nd 1/2 original tax bill (Builders cost, lot taxes only) and figures they've over collected. In this case, do not spend that check because it's an incorrectly figured refund. The County will reassess and notify the lender who then will ask for $$$ to pay the County.
2) You buy for $1m. Your property taxes are calculated at 1.6%. The actual taxes are 1.4%. The refunded difference between 1.6 and 1.4 is the amount you've been given. Don't spend that check, because the initially billed Mello Roos might be higher once the County gets all of their data correct, sometime in June.
3) You buy at $1m. Your property taxes are calculated at 1.6%. The servicer gets the bill for the supplementals, but not the 2nd 1/2. (yes, this does happen....) believe they are over collecting, then refunds the difference between the 1.6% collected and the small amount on the Supplemental Taxes. Don't spend that check either.
4) You buy at $1m. Your property taxes are calculated at 1.6%. You close in late December. Your property taxes are correctly balanced in your impound account. In this case it's possible that the seller/builder paid the 1st 1/2 taxes on December 10th, just as you did at closing. The refund might be the overpayment of the property taxes due. Don't spend this either because the seller may be due a refund and Escrow would need to be involved to sort things out.
It's not a matter of impounding for taxes and insurance as being problematic, but rather so many people involved at closing where mere errors are likely to be made. Once the County reassesses in June and the Supplementals are paid, the chances of additional impound account errors diminishes by quite a bit. If it becomes too much of a hassle, you can usually (not always....) cancel your impound account.
Word to the wise. I've seen situations where Supplemental Taxes aren't paid on time, and the servicer will not allow impounds to be removed. Why? Taxes take priority over every lien, and if you've shown that you can't pay your taxes on time (perception = reality) then the lender has a right to retain impounds, even if your LTV is low.
My .02c