Why were you assuming a particular tube current would result from factory voltages? Tubes vary a lot tube to tube, so there is no voltage you can set that will always result in some particular current. The reason you got 13ma idle current is that that is what that particular set of tubes runs at with the given voltage.
There is no "right current" in the tubes. There is what works well, and what does not. Nominal is just that, a named amount acting as an average point of reference.
13ma would be dead on average for a Peavey 5150 stock amp, they all run cool.
There is nothing precision here, it is just a guitar amp. Remember, your B+ voltage will change almost 4v for every volt the mains shifts. SO if mains goes from 120v to 125v, your B+ will rise close to 20v. And that certainly affects tube currents. The bias supply also grows at the same percentage, but the effect on currents does not track that.
When I step on stage, it is my ears I aim to please, not a schematic. This schematic lacks the note, but MANY Fender schematics note right on them that voltages presented can be within +/-20%.
70% is a rule of thumb from the internet, but is not designed into the amp. There is no way to engineer a particular current into it without a complex current sensing servo circuit.