HTupolev

They don't all have a 34T max; the RD-RX812 has a 42T max

The RD-RX812 is a 1x derailleur in two ways.

First, it's only designed to wrap enough chain to handle a 42-11 cassette, which is 42-11 = 31 links.
Whereas an RD-RX810 is designed to handle up to an 11-34 cassette along with a 48-31 crank, which is 34-11+48-31 = 40 links. To handle this, the RD-RX800 and RD-RX810 use a longer tension cage than the RD-RX812.

Second, on the RD-RX812, there's a lot of distance between the tension cage pivot and the jockey wheel. This causes the position of the jockey wheel to be heavily dependent on how much chain is currently being wrapped. When the drivetrain shifts to a bigger cog, the tension cage swings counter-clockwise (when viewed from the drive side), lowering the jockey wheel. This means that the derailleur will actually adapt itself to the cassette somewhat; if you switch to a smaller cassette, the jockey wheel will stay close to the cassette when you're in the now-less-big big cogs, so shifting won't degrade much when you switch between different cassettes.
HOWEVER
Front shifting ruins this scheme. When you shift to a bigger chainring up front, the rear derailleur swings counter-clockwise and pulls the jockey wheel away from the cogs, affecting the rear shifting. This obviously isn't desirable. So, the RD-RX800 and RD-RX810 keep the jockey wheel close to the cage pivot, so that front shifting has a minimal effect on rear shifting. Instead, they rely mostly on the geometry of the parallelogram to manage the distance between the jockey wheel and the cogs.