Strange voltage readings, any explanation?

[Ben Nevis]

At home on hook up for 36 hours with fridge on 4.
Checked panel 13.2V, -0.2A so assumed fully charged.
Unhooked from mains & checked panel 12.3V, -0.2A but battery shows only 60%.
Started engine & checked panel 14.4v >20amps with generator symbol on.
After 2 minutes stopped engine checked panel 12.4volts and battery shows 70%.
On hookup again for 10min, checked panel 13.2V, -0.2A
Then unhooked 12.8V, -0.2A battery now 100%.

Any explanation for the low 12.3V 60% reading when it should have been fully charged, or does it indicate a fault somewhere that should be reported to the dealer?