Hi matt, certain diffusers require different pressure in order to diffuse properly. The bubble count only controls the amount of co2 you want to go into your tank not the pressure at which it is pushed through at. If your reg is set at 1.5 bar pressure, you'll still have that going through whether your at 2 bps or 5. Slowing the bubble rate doesn't alter the pressure through the tubing.
Yes, the more co2 flow you have the higher the bubble rate but that still isn't related to the amount of pressure.
Take a balloon for instance, now firstly try blowing that balloon up very gently but at a steady rate...it wouldn't expand properly right? Now use more pressure by blowing harder but at the sane rate and the balloon fills properly. That's how it pretty much works.
Inline diffusers have such tiny holes to produce the mist that they need a higher pressure to force the co2 through