That's very unusual Julian. It's more usual for an NiMh battery to refuse to accept a charge if it's been left months without a charge and the cell voltages have dropped to very low levels. However, that's not an absolute.
I think what might have happened in your case is that the hydrate has recovered and normalised without losing all it's hydrogen content, that enabling a recharge.
NiMh are actually a form of fuel cell, but a closed circuit one. The fundamental principle of their working is that charging hydrolyses water, turning it into free oxygen and hydrogen which combines with rare earths, forming the hydrate.
When connected to a load, the hydrogen is released from the hydrate and recombines with the free oxygen, releasing the stored electrical energy. Your rested and revitalised hydrate has given you more range, but it might be a bit early to celebrate it since the gain might be very short lived, returning to the short range you had before, or even less.
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