Have you checked pentaprism manual? You can find it online. There is a chart in the manual pointing to certain ISO Shutter combinations not metering accurately. May be your exposure was in that range, however unlikely it is worth checking.
But the battery might be the culprit.
Although the pentaprism readings are accurate in most conditions.
I have noticed that sometimes it gives weird readings in tricky situations(too much light or too dramatic lighting).
When I am in doubt:
I switch off reader point it to the ground or some place that has more homogeneous light, turn it on than point it to the scene I am metering.
I usually get better readings this way.