Normally that's true. The bec side can/will use 100-1000 more mah than the other side. My guess is that the cell that went bad is the smallest/weakest cell and may or may not have been charged as much as the pack that powered the bec side.

Mine are absolutely consistent - the bec side is down 400-700mah from the other batt over a 15-17min run. When I was running NiMHs (7 cells packs, so they were "handed"), I used to wonder why the left one would always take 500-900mah more when re-charged. My lipos show a little bit less diff.

If his lipo experienced a prob, that would explain things and make sense.

Mine were always consistant too (at around 200-400) with 3300 mah packs. The way I drive don't use a lot of steering though. I can easily see someone who pots around a lot with a strong servo using over 1000 mah more on large capacity packs.