The decoder monitors each transponder's multiple transmissions ("hits") as it passes over the loop, and tags each hit with an amplitude and time. Then it compares all of the amplitudes for that transponder's hits, and interpolates where the maximum would have occurred in time if the transmissions were continuous. (Imagine creating a graph of the signal level of each transponder as it goes over the loop, drawing a smooth curve through the data points to find the peak.) This is the time it reports to the lap counting PC. The overall amplitude of the signal is unimportant, as long as it is strong enough to give multiple hits.

It is this interpolation that permits an accuracy that exceeds the repetition time of the transponder transmissions. That's pretty slick!

The signal strength is unimportant as long as multiple hits are recorded. Even if it were, the transponders are regulated internally, so the strength is the same with 6V supply as with 9V supply.