There are nouns that are masculine in the singular and feminine in the plural:

the finger -> the fingers

the egg -> the eggs

the arm -> the arms

the mile -> the miles

the pair _cc781905-5cde-3194-bb3b-136bad5cf58d _-> the pairs

​

But why? It's a long story, and it has to do with Latin :-) For example: OVUM plur .: OVA ...

​