Other user's profile picture keeps changing

[w1nter_proxy]

Is it just me or other user's profile picture keeps changing?
Every time I reload the page on forum, some of the user's profile picture keeps changing.
I'm not complaining, I just want to know the reason why there picture keeps changing.

Is it because they didn't set their profile picture?
Or is it just me seeing things?

 

[Granitefish]

If someone has not set a Profile Picture it will use one of the Default Images which are not set and there are several of.

 

[Holo]

The default avatar rotates through a few images.