I suspect there is not a definitive answer. While the amount of charge of a battery is a definite, known quantity, passed to the computer by the chip on the battery, the estimate duration of a battery charge is done by dividing that quantity by the average energy consumption for a given unit of time.
It might well be that your debian and ubuntu installations have a different record of your battery usage history (e.g. they have a different record of how much energy you typically use in one unit of time).
In some cases it's possible to make the measurament more accurate by actually doing a couple of complete cycles (full charge -> full discharge) while using the computer normally.
An experiment you can do is to actually check if the actual duration of the battery is the same under debian and under ubuntu. Should you find that is not the case, than you would know it's not a difference in the recorded data, but some difference in the system configuration that brings to an actual different power usage.
If you will do any of these test, I would be very much interested to hear what was the outcome, as I am a GNU/linux user (various flavours) too!