It *DOES* update the existing record, *IF* it can match it.
You get the same issue (matching devices) with bare-metal scans which - for all intents and purposes - are the exact same concept. That being - "very limited data scan file".
The matching happens via the MAC-address primarily ... and/or failing that via a serial number.
Now - Serial numbers may "vary" ... you may have noticed that we've got serial #-s in multiple places for instances - depending on the (in-)competence of whoever did the BIOS for instance, those serial numbers are all consistent (yay), or in especially fun situations, you end up with 3 different (and/or differently FORMATTED) serial numbers in different places.
So the good news is that - by and large - this is very simple to observe. You can just turn on "Store scans" and then re-play scan-files after deleting a record to review what's happening.
It's possible (for instance) that your MAC-address comes in as "AA-BB-CC-DD-EE" format ... but we strip MAC-addresses of any ":"-s or "-"-s and would just have "AABBCCDDEE" for instance. So - have a careful look at what attribute you're expecting to "marry" a record off of. Chances are more than likely that there's a mismatch (compared to the full scan file) that's causing the Inventory Service to go "Nope - that's someone else".