It is possible that people grow over time and their post quality might improve as a result of their growing as a person/forum user. I say it's possible, but obviously there are other scenarios that might be in play here.

1. Account sold. New owner trying to cover up old posts maybe because old owner was a regular poster in Nigeria local, new owner Indonesia or whatever different locals.

2. account hacked, and hacker is trying to clean up the account for resale

3. former spammer trying to make the account look better to join a sig campaign

I'm sure there may be other reasons, but these are what come to my mind.