VA Home loan through USAA?

I am currently active duty Air Force and looking to buy a home. My price range is under $140,000. I called USAA who I bank through to get a VA loan and they pre-approved me for $140,000 @ a 30 yr VA loan. Their website says they only do fixed rate 30 yr VA loans and the current rate is 4.25%. After running the... show more I am currently active duty Air Force and looking to buy a home. My price range is under $140,000. I called USAA who I bank through to get a VA loan and they pre-approved me for $140,000 @ a 30 yr VA loan. Their website says they only do fixed rate 30 yr VA loans and the current rate is 4.25%. After running the numbers based upon the listing for the house im interested in (which includes the taxes $1385) and getting a quote through USAA for home owners insurance for the type of house I'm interested in (~$40)... I came up with ~$675.00 (Principle, Interest, taxes, Insurance).
My pre-approval letter says the PITI is ~$868.00.
How can I be so far off from the approval letter? Are their any other factors that I could have forgot?
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