Two minor notes
@Ian Doncaster : first, the FSS and DSS were added in 2018. Dec., so a year earlier than the dates you wrote. Everything from 2019 was FSS.
However, my second note is more interesting: the level 2 scans that you mention, which were without a DSS fitted, would not only not give you the full payout, but they also wouldn't give you the full characteristics of the body you scanned. Some of them would be missing if you viewed the body on the system map. (I can't remember if they were always the same characteristics, or if there were different ones missing different times.)
This was by design, mind, and the reason given for a DSS scan paying more was that you sold more information to Universal Cartographics. Well, since an unengineered DSS weighed only 1.3T, almost everybody fitted one anyway. (A few record breakers who made completely stripped cardboard Anaconda builds didn't mount one.) So this was mostly a curiosity, except for those rare times when parts of the data uploaded to EDSM were missing - those were annoying.