Your vertical metrics values exceed the UPM (901+250=1151) and this behaviour would indicate that you have not balanced the metrics accordingly. If you read through Karsten Luecke's guide to font metrics carefully and adjust your settings accordingly you should find that the vertical metrics start behaving as you would expect. At the very least you will have a greater understanding of what those settings change.

Additionally, the Typophile threads listed below all deal with this issue in detail:

http://typophile.com/node/77906
http://typophile.com/node/13081

If I read correctly, 901 is the maximum ascender value, i.e. WinAscent, not the TypoAscender (which the OP does not mention). I've always understood that it's TypoAscender and TypoDescender that should fit within the UPM, not WinAscent/Descent.

Typogruffer, what is your actual character ascender value, i.e. the highest point of any base (non-accented) character? And do your OS/2 and hhea table metrics match it and the other values mentioned (as described in the referenced PDF)?

Hey altsan, my TypoAscender value is 750. I have to modify my metric values using Karsten's PDF as a guide. I will get back to you with the results.