Restoring from a Time Machine back-up - puzzlement!

Hello - I've read here: https://support.apple.com/en-gb/HT203981

Yesterday, I decided to wipe the hard drive on my iMac and reinstall Catalina from my Time Machine back-up. When I had to choose WHICH back-up to install, I noticed something which I felt was rather odd. Instead of simply choosing a particular date/time of the back-up to use, I had a choice of which hard drive to select too! There were many more dates available on one of the disks than the other. I took a photograph of what I saw to show what I mean:-

Have you ever encountered such a thing before? HD and HD1 - as I’m sure you can see.

SHOULD it be like this? Is it a phenomenon of macOS Catalina perhaps?

I decided NOT to use my Time Machine back-up at all. I did erase my hard drive ……… and then did much ‘fiddling’ to get Catalina back up-and-running again on this machine. Everything seems to be working but I’m keeping my external hard drive in its original state for now - I’m NOT making any back-up for the time-being,

I’d really appreciate your thoughts on this. Thanks.