Probably means the battery was already half-charged. I don't know if it's the same with all chargers (only recently got into fancy chargers) but it'll put in as much as there is room for and then trickle or stop the charge. This provides a good indication of how much has been drawn out of your batteries in a battle.

For example, if you have a 3000mAh battery and it only charges to, say, 1000mAh after a fight, that's how much you took out of it in that fight. So your battery still had two-thirds of its capacity remaining.

At least I think that's how it works...