Unless their client repackeged the wrong SSD inside the metal box, that isn't the drive you were sold. That secure erase theory won't even work here, because even if that previous client did a low-level over-provision, a secure erase wouldnt change its capacity. It also doesnt add up that not only is the capacity less, but after Windows was installed there is only 83GB free.
There are so many red flags here that it boggles my mind.
Right now I really, really want to see what is inside that SSD case. I have a strong feeling someone messed with the partition mapping of a smaller capacity SSD to make it look larger.
Also the part I'm really unclear about is why this SSD was purchased used in the first place now that you mention it. Are you not located in the US and are just dealing with higher international pricing?