What client is that? While I agree in principle with your math, we're now in a situation where there is a 100% chance we have seen a bug. Either the bug is in the description of the quiver (by calling it DM, instead of DI), or it is with the way that client calculates damage.
I can tell you that both of the OSI clients do NOT update the character window - not for a quiver of infinity anyway. With 150 str, 120 tact, 120 anat, and 100 DI from items my OSI clients showed me at the same 64-80. When I put on the quiver it didn't change. When I took off some DI from items(by letting DF expire...) my damage shown went down accordingly, but when I added the quiver back in and out, it didn't change.
One of our clients, therefore, has a bug. But we still can't say for sure whether the quivers really add DI or DM without testing.