Thanks. Without knowing the details (volume, assets involved) of the trade that happened at 1.5 it is impossible to tell if the rounding issue is the cause.
What I take from this is that we're talking about a UI issue here. Your point that this is unexpected and leads to confusion is absolutely valid. I do not see this as a bug in the market engine, though, but merely as a display problem.
Edit: added comment to github issue https://github.com/cryptonomex/graphene/issues/460#issuecomment-236355266