The same thing -- i.e., a change in pronunciation in a different form of the word -- happens in English too, although it's not necessarily reflected in the spelling (because English is such a messed up language, orthographically speaking). A few examples: child --> children; woman --> women; louse --> lice; thy --> thine; vine --> vineyard.
This is just an irregularity. It emerged some time ago and stayed in the language. It could be for various reasons, for example easier/more natural pronunaciation. Maybe this helps: