Recuva vs. - each found an entirely different set of recoverable files

I used Recuva and [edited by moderation] on the same drive and each program came up with an entirely different set of recoverable files. None of the files found by Recuva were found by [edited by moderation] and visa versa. How is that possible? Thanks.

It isn't allowed on the forums to discuss or compare similar or competing products vs Piriform products. Of course you wouldn't have known this.