Simplist take on the URT (IMO):
Think of the swingarm as a simple lever, riders mass when stood up is applied at a point much closed to the pivot than the rear axle which under compression moves relatively less than the rear axle, but of course you are still on the same frame member so while you are not utterly removed from rear axle you are experiencing less of the impact/vibration(s).
When seated more of the riders mass is applied direct to the pivot actually making the swingarm more “active” but pedaling a bit more of a bouncy affair, firm up the spring to make the seated behaviour of the frame more acceptable for pedaling and you knacker the out of the seat operation of the suspension.
If you look at the extreme example of a URT; the ST10 that is realistically going to see most of its use with the rider out of the seat, and hence is normally sprung a lighter than an equivalent “Fully Active” design might be. It delivers some key advantages; simplicity, Zero chain grow (so could be SS’d, Alfined, geared), less pedal bob (When stood up) and arguably more “Feel” for the back wheel. But of course it’s not perfect (no bike is) when seated the suspension will behave very differently, where and active design will be a bit more consistent across all rider positions, it works in that instace as the bike has a rather specific purpose, but for an “AM” or XC bike where climbing and descending performance are both important a URT might well not get the balance of compromises quite right…
The GT/mongoose I-drive type designs might appear URT-esq but they are in fact designed specifically to articulate the BB and separate it from the rear axle/swingarm motion, it ain’t a URT and it isn’t a single pivot, faux bar or Horst type design, it’s something different again.