Hi Meliffy,
Last year I had the same experience at first - that the predicted weights were a little too heavy (usually for upper body) or a little too light (lower). It was easy to adjust.
In hindsight it's pretty clear why that was true for me, though who knows if these reasons would affect you as well. First, I had at least as much a psychological difficulty with heavy weights as I did physical! I'd never used a barbell, or really heavy dumbbells. So, when I did the 1RMS, my mind convinced me I should stop sooner than my legs did

Also, my arms and grip toughened up fast, and sometimes when I finished a 1RM testing session it was because of losing my grip and not because the body part was actually tired. That's not something I knew to look out for at the time.
For upper body, the 'problem' was that I had no endurance at first. When I did the 1RMS, I was fully rested for each one and so my predicted weights were what I could lift fully rested ... but of course, I wasn't fully rested midway through

It gave me something to work towards, though, and I adjusted both online and during the workout so that I'd be just barely able to complete each set. Sometimes I'd just pause, other times I'd do my best. By the time I started Meso2, the predictions were spot on.
I'm very glad I did the 1RMS; they are a great record of where I was when I started the program, and how much I've improved. Hopefully you'll also find them to be a good investment of your time.