that "regulation" you see is more likely due to the constant voltage that Li-Ions can hold rather than the light being regulation. The much longer runtime is a tip off, there is no way a 17670 or similar sized battery can put out more capacity than a pair of primaries, especially when moderately driven.
My G2L runs noticebly dimmer on one li-ion battery than primaries. It also depends on the Vf of the LED, but in general a Li-Ion battery would give shorter runtime than primaries unless it was a high-drain situation, in which case the P60L is definitely not.