Yesterday I had 10 maximum flags. I flagged two posts for attention: one to be migrated to meta, and one that wasn't really a question. Today I have 12 maximum flags. Was I credited with a higher maximum for my actions, or was this something else?
It's because your flag weight increased by 40, probably as result of good flagging.
See this post for the full details: Are the new moderator flag limits sufficiently high?
The relevant line is:
one bonus flag per 20 "flag weight" above the default (capped at 500 flag weight)