Languages evolve over time, When a new character was coined for a simple definition, it only represented that particular definition.
By extension of logic, additional meanings can be added to that character.
革 was originally a verb for "remove animal skin". When people needed a word for ' leather', they put 皮(skin/ outer layer) and 革(removed skin from animal ) together and coined the word "皮革".
by association, the word "皮革" also gave '皮' and '革' the meaning of ' leather'
When people needed a word for 'cast out' they just borrowed the 'remove' meaning from 革, (to cast out someone, is to remove someone from his position) broaden the possible object from a specific one 'animal skin' to indefinite ones, thus 革 also carry the meaning of 'cast out' and 'remove'
變 means 'change'
革 means 'remove'
變革 means 'change' (by remove existing system or rules)
改 means 'alter'
革 means 'change'
改革 means 'reform' (by alter and remove existing system or rules)
Most characters went through evolution like this. I can list virtually thousands of examples. Take the character '日'. It was originally coined to represent only 'the sun'. Since people were using sun raise and sun set to mark passing of days, the character '日' was naturally given the meaning of 'day' as well.