My experience is similar in that the charger does not seem to provide the equivalent of 5 full 0 to 100% charge cycles to the earpieces in my typical usage which should be 10, 50 to 100% cycles.
I would hazard a guess that the latter 1/2 of the charge cycle is less efficient than the first half. For illustration, at 0% charge, 100 electrons from the charger results in 90 stored in the earpiece cells. At 50% 100 electrons from the charger results in 70 stored in the earpiece. At the end of the charge, the same 100 electrons might only result in a few stored as charge in the earpiece cells.
Furthermore, with the Dash not being in storage mode, there would be a standby current drawing down the Dash charge that requires the charger to provide a float voltage to maintain the earpiece charge state which would reduce the number of charge cycles available. Over a period of two weeks, I was on one occassion only able to get a bit over two cycles from the base charger and guess that is the most likely explanation.
I wish the documentation were clearer on this point.