Because as long as it isn’t overwritten it can sometimes reside in a residual way in the storage sectors on the drive, these hdd scanning software’s check through the sectors for data hiding in them some sucxessfully some not as successfully, there for some will find more or less data than others do as well.
This is why data disappears on drives as well when a physical issue causes the sectors of the drive to begin to stop working aka “bad sectors” this makes the data start to seemingly magically vanish or corrupt if it’s still operating and booting into Windows you can at times witness the data/folders and or files present in folders one moment and missing fron the OS the next, that’s an indictator often of an imminent drive failure due to bad sectors. In this scenario it get’s less likely you’ll recover the data the longer the drive is in use because more of the sectors will probably die. You want to be doing the recovery and not using the drive in Windows in this instance. I say Windows but it applies to any HDD with any OS installed really.