Oh it totally makes sense - the thing that surprised me was how it was called weight on the form, so I just copied the weight across from the parcel I had received. I'm sure it was partially user error, but UPS could certainly have done more to make me aware that there was a difference.

If you're sending parcels regularly, I'm sure this is all well known, but this was the first time I'd sent a parcel myself (instead of just going to the post office and have them handle it).