Don't forget that people often don't believe the auction catalogues. They may have not realised that there are fakes out there, and just thought that the auction house didn't know what it had - this is more common than auction houses knowing after all.
If the vendor doesn;t know what they have got, then unless it is a specialist auction, the likelihood is that it will be mis-catalogued.
So perhaps they thought it right, and bid accordingly.
And didn't like the real one as much!