What you are seeing is one light at a time, but changing quickly so it looks like more than one is lit. If the voltage is at the cusp of the change from one LED to another, it may flash back and forth quickly looking like two are lit.

Once you provide a load, or a changing load (ie. servo's moving), the current will increase and the voltage will decrease (slightly) which is what is being detected.

With a fully charged battery, the voltage is somewhat higher than the rated voltage... 4.8 volt pack yields about 5.4 volts at full charge. Same is true for 6 volt (5 cell) packs which yield about 6.8 volts when fully charged. Note I said ABOUT. So, don't jump my case if this is not absolute.

Once the battery is connected and starts to discharge by usage, this voltage will drop to a point where it reaches a plateau where it pretty much stays as the current draw continues. But, once it reaches a low point of current flow, that plateau drops quickly as the battery reaches full discharge. That's the point to avoid. Stick with voltages in the plateau, around 4.6 to 4.9 volts.

Remember that current draw is different in every situation. As you add items, another servo.. or move more servo's at a time, this current draw increases as the load demands it.

What I am trying to say to you is to watch the voltwatch (for instance) and move the sticks. It will drop, of course, but make sure it does NOT go in the red zone. It may digress into the yellow zone for a moment or two while the servos are working, but should not stay there once you stop. You should not fly if it goes into the red zone. You are approaching that plateau I just mentioned and it could drop down quickly (as NiCd's and NiMH's will do) and you will be in serious trouble.

I am not quite sure if I believe 15 10 minute flights out of a standard receiver pack.

On my larger planes, I am using a pair of 1200 mah NiMH 6 volt ( 5 cells ) packs. Each one has it's own switch and each one goes to a separate vacant channel on the RX. I told a tale in a previous post of a problem that developed with my Venus II that the dual battery setup saved. I won't go through that again, but I will say that it saved my Venus II.