This has evidently been confirmed by Microsoft, and there is a fix on the way. For more on this, check this link:
It appears to be a display only bug, and the calculations are actually correct, but if you look down in the comments, you'll see it gets even more confusing. Looking at the results below, this gets pretty wierd:
A1 =850*77.1 --> 100000
B1 =A1+1 --> 100001 (realy a display bug???)
C1 =A1-1 --> 65534
D1 =B1-C1 --> 2
Anyway, be on the lookout for the fix, because it's in final pass testing!