The weirdest thing to me is that the Formula Four 101s can sometimes be more slick with some sets and sometimes be more grippy with other sets than the standard 99s, which seem to always have the same feeling from set to set, regardless of size, shape and how old they are - all factors which definitely make a difference with other wheels I have owned over the years.
Other people have said this too.
If anything, it might be that once you get into a slide, the 101s can sometimes slide further and longer than the 99s, but the most recent set I got from someone just didn't want to slide much at all, so they ended up on the "slippery surface board" and had more grip than the new 97s I had just put on.
Either way, they are great wheels and I am not about to change any time soon, but I prefer the 99s over the 101s and have had the 97s for rough ground or when a bit of extra grip is needed.