I don't know the scientific explanation. But to me, our bodies adapt to our surroundings. Whether that be our environment, our sleeping pattersn or what we eat. I think it's healthy so mix it up every now and then, and clearly it's worked for you! Way to go! That's an awesome achievement.
For me, I'm so used to running off little sleep, and when I get more than 'usual', I'm more tired, groggy, late for walk etc. And when i get more, my total hours of sleep equal that of a 'normal' person. I'm trying to break that, but it's hard.
Now that you've smashed through the plateau, hopefully you'll see some more progress. Congrats!