Yes, I have noticed this as well.
My theory is that the stats are generated on-the-fly whenever you access them, but there is a hold-off period of 24 hours after each stat generation. So, if you have not looked at your stats for the past 24 hours (and no one else has either) and then you look at them, the is no running hold-off period anymore and you get the updated stats immediately.
I believe this is a separate counter for the Profile Stats page itself and for the image-generated stat you put on your profile.
Just a theory off course, I have no insights in the actual code.