There are three possible reasons for this:

1. SpamCop's code has changed
2. One of the databases that SpamCop uses has changed
3. The spammer has changed providers for their website

SpamCop is an ever changing entity in a moving sea of data. Not only does it use internal databases to keep track of contact emails, closed accounts, historical data, etc., but it also relies on network databases to be up, operational, and accurate.

To add to this confusion, changes are frequent to the algorithm, so you may experience a bug one day and find it fixed the next. Unfortunately, the reverse is also true - everything can be fine one day and new bugs appear the next. Bugs are usually minor, and you can have the fun of being the first to report a bug if you are quick.

Whenever there are major changes, SpamCop gets a new version number, but many changes will appear without a change in version number.

Becoming more common, a spammer who owns their own domain name may change hosting services, by choice or not. This is why SpamCop may recommend one abuse address one day and a different one the next day.
[Append to This Answer]