253 online challenges complete but no achievement? Why?

Okay, so last night, I finally completed my 250th online challenge....then my 251st, 252nd, and 253rd. I kept waiting for the 250 online challenges achievement, but it never happened. Are there any exclusions that prevent some of the 490 challenges from being used for this achievement? I know that a few of my completed challenges were timed challenges and bike challenges.

Thanks for any help you can offer. Even though this is my first post, this forum has been incredibly helpful to me over the past 2 months of my Burnout Paradise addiction!