I am pleased to hear all went well and that you decided to even go for a third attempt, Bravo. Unlocking the phone does Wipe userdata.
Not sure I am the best person to ask and I did not find a quick answer with an online search. I did find this image in the article linked below it.
In short the idea is to have two partitions of the “hard drive” with the aim of making an update more secure. Slot A is flashed one day, if there are bad consequences, the phone could revert to Slot B. A far better explanation form XDA developers above. Just a guess, but I expect the “factory install script” may be quite hard work for a home PC, made harder if the PC has to drive Easy Installer as well (??).