Well, no one would say 'My fingers are broken' meaning their toes in Spanish, you would say 'The foot's fingers are broken" or 'I've hurt my foot' and then specify what's happened. They are different things alright, that's why one is a compound word and the other isn't.

I found a worse offence that English uses 'the day before yesterday' instead of a word, in Spanish we have 'antier' or 'anteayer'.