A car has an onboard converter that converts AC power (alternating current) to DC power (direct current). This is because the grid provides AC power and a battery uses DC power to charge. In this conversion process, as in any process, energy is lost.
In some cars this loss is around 10%. A car continues to consume power when charging, so when the percentage of the battery drops below a limit it starts charging again.
So it may be that your session at a charging station has consumed more power than the capacity of your battery pack can handle. However, this is nothing to worry about.