After a lot of thought and observation I have a deduction about the P893 that explains some stuff FoxKat and I were curious about about 6 months ago.
The P893 works so good because it puts juice on the 4 pin on the micro end! Just like a moto factory programming cable. Bypasses phone battery entirely! That's why the indicated charge state doesn't change until the P893 dies AND lets the P893 kill itself completely, help a bionic (or razr, or D4) recover from a totally killed battery so quick, when you daisy chain wall wart to P893 to phone.
I haven't been able to test because I don't have a probe small enough to hit that 4 pin, but it's the only remaining hypothesis after the impossibles are eliminated, so it's gotta be.