Battery charge

Battery voltage only gives you a useful reading if the battery is not hooked up to anything. If there is any drain, at all, the voltage will drop in proportion to the draw, and give you a false reading. If you want an accurate reading, you have to disconnect one terminal and give it 5 minutes or so to rest/stabilize. If you tested the voltage while the battery was still connected to the converter, there are draws, not least of which is the converter itself.

If your test was done with such a disconnection, now you do have a mystery.