1. Maybe, or maybe an erroneous message. Perhaps there was an issue recorded with it during the previous flight but the error had cleared.

2. 'Any reason is always more credible than 'these things happen.'

3. The flight may have been full and upgraded from WTP, or they may have been on staff travel/standby.

4. They may have taken the last two seats in F (and been on the upgrade list for those even as you were boarding). They may even have been the former occupants of 20K. If there are empty seats in CW and F, the F seats should stay empty, so it's likely that they were making way for people who'd themselves been upgraded.