This can be due to a number of factors:
- perhaps one or more RBLs are offline/unresponsive during the time you run your blacklist check
- perhaps your blacklist monitoring report is older (even 1-2 hours older) and your IP got delisted in the meantime
- it may be due to the fact that blacklist check is not as accurate as the blacklist monitoring system
You can read about the differences between these two here: Blacklist Check vs Blacklist Monitor