The easiest way to address this would likely be if you sent me (
di...@subsurface-divelog.org) an email directly FROM the address that you use to log into the Subsurface Cloud and give me permission to access your dive data.
Then I can go in and see what's actually in the dive log and why you see that discrepancy.
My first guess would be that you have two dives that aren't numbered, so the numbered ones run from 1 to 51, but there are two more dives with number 0.
Is it possible that you didn't delete them but marked them as 'invalid'? That way they wouldn't show when you look at your dive list, but they would still be 'there'.
/D