As I recall, Kiva mentioned a few different options they considered for gating the progression of mechs encountered, but I think they settled on just using the mission difficulty (including the +/- 1/2 skull actual difficulty variance bit). I think the MRB rating was probably implemented before they finalized the patch, but that tool tip made it in.
I will mention, though, that you may get a higher tonnage mech with armor debuffs or a single higher tonnage mech in place of what would normally be two lower tonnage mechs (ie, a battle mission vs a lance with 1 heavy and two mediums in a 2 ish skull mission). There are also two tiers of armor debuffs, so you can get a much higher total tonnage OpFor but with paper thin armor as well.
Some of it also seems to fall into "cultivated procedural content" based on the mission such that the random possible enemy lances are sometimes skewed towards one of those factors above as well.