C-Pap vs Bi-Pap

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Someday this may all make sense, but for now I'm waiting for my PR Bi-Pap.
At the risk of starting off in yet another direction, I don't understand why the IPAP and the EPAP aren't always different and if so, why the Bi-Pap is not the standard? (Or was that issue already explained in terms that flew over my head?)

Fixed-pressure CPAP machines came first, meaning there was no pressure difference between inhale and exhale, except the exhale pressure tended to be at least slightly higher (which was opposite from what we wanted).

After a lot of R & D investment, bi-level CPAP machines were released which allowed IPAP to be higher than EPAP (a lot higher, like 8 or 10 or 15 cm H2O higher), but the bilevel machines were more expensive than the fixed-pressure machines. They still are more expensive, even though the hardware cost is (I think) not much different for a fixed-pressure model versus a bi-level model. The difference is mostly in the software, but the blower unit also needs to be more responsive and would be more expensive. IPAP = EPAP + Pressure Support. The PS boosts the IPAP pressure above EPAP.

Recently (within past 10 years or less) many Fixed-pressure PAP and auto-adjusting PAP machines now allow EPAP to be lowered a limited amount below IPAP, generally not more than 3 cm H2O max. This is called EPR or Flex or EZEX or whatever, although there can be differences between these, as was explained by RobySue. This is used by most patients. A minority find they do better when the pressure is the same (or nearly the same for EPAP and IPAP, so these turn down or turn off this feature. EPAP = IPAP - EPR. The EPR (or Flex or EZEX or whatever) drops the pressure EPAP below IPAP.