This doesn't seem very normal to me:
Along the same lines:
We're aware of the issue, but we're not sure what exactly is going on. As far as we know, it's not something on our end. Gravatar seems to be serving different identicons for the same hash but different requested sizes... and only sometimes.
Marc Gravell wrote up some more details here: http://meta.diy.stackexchange.com/questions/1218/hmm-something-funny-with-the-what-do-they-call-it-gravatar/1219#1219