More 'updates' than 'units' on App Store

Hello all - I have undertaken the (fantastic) CWC course, and I’m very happy to say that, as a result, I was able to launch my very own app on the App Store as a result (hat tip to Chris!)

I had a curious question for anyone with experience of the App Store:
I released my (free) app just before Christmas, with an update to it released a few weeks later.

Attached are two screenshots from the App Store Connect app (showing metrics for my app) which shows a total of ‘54 Units’ (which I presume means 54 installs’), but 74 Updates !!
I can’t for the life of me figure this out: how can there be so many more ‘updates’ carried out, than actual installs. The data is for the same 13 week period (26.Oct.20 - 24.Jan.21) … just thought I’d put it out there, in case anyone has any clues: how can there be more ‘Updates’ to ‘Units’?