Wrong remaining capacity of my battery when it is charged

Hello,
I have a problem with the remaining capacity indicator of my battery. Here is what I read in /proc/acpi/battery/BAT0/info:

design capacity:         79920 mWh
last full capacity:      75000 mWh

I run Linux 2.6.15.
But when the battery is charged, remaining capacity as indicated in /proc/acpi/battery/BAT0/state is 79920 mWh. When the battery is charging, remaining capacity slowly rises to 75000 mWh, then jumps to 79920 mWh. I could not figure out if remaining capacity is computed by the battery itself, which would mean that there is a little bug in my battery firmware, or by ACPI, using other informations. Thanks in advance for explanations!
Nicolas

ACPI does not compute anything. Information is supplied by BIOS methods (_BST in DSDT). _BST could compute something, but it usually just reads registers of embedded controller (battery).

Regards,
Alex.