I had noticed that my profile picture also changed automatically this week. The following screenshot combo is showing the difference for my profile picture (upper half showing older ones).
Similar to OP, I have been:
using auto-generated pattern image and never uploaded any personal picture;
using same login method i.e. Google account with same e-mail address;
using same computer and same location i.e. from home
Initially, I thought...
Then this week, all of sudden, I realized that my public IP address has been assigned to significantly different ones. While it is known that the public IP address could change from time to time [1,2], it is the first time for my computer to be assigned with different range of public IP address.
The difference is significant such that it affects my login experience on Ask Ubuntu (profile picture changed) and Google Mail (precautionary two-factor authentication was activated).
Later, I figured out...
I reviewed this FAQ post on meta SE, regarding the default profile picture (identicon). It seems that IP address is not relevant for registered users (including me).
When you're not registered (when Stack Exchange does not know your email address), this is based on your last known IP address so might change when your IP address changes
Then, I found this related old post on meta SE. The OP included a URL of hashed e-mail address that links to Gravatar site, as following:
So I tried to reproduce the current profile picture (identicon that has more patterned geometry) by running
echo -n YOURADDRESS | md5sum in Terminal. Replace
YOURADDRESS with actual e-mail address i.e.
-n option is required to remove the newline.
$ echo -n firstname.lastname@example.org | md5sum
Then, copy the checksum and replace
[md5sum-of-email-address] in the Gravatar URL.
Finally, copy and paste URL into web browser to view the identicon.
And, bingo! I got the same identicon as my current profile picture.
What does this successful reproduction of identicon mean? Most likely, the changes had occurred on Gravatar site, and not Stack Exchange sites. The changes might have something to do with undisclosed algorithm to generate the patterns, without altering the hash of e-mail address itself.
TL;DR It was nothing to do with IP address for registered users. Something has changed on Gravatar service site, which caused pattern generation to be different compared to older ones.
Why does my public IP address change?
Why Do ISPs Change Your IP Address?