Chalk and Cheese
Chalk and Cheese : Phrases
Two things that are might possibly be confused but which are in fact very different.
There are several phrases that suggest one thing is interchangeable with another, e.g. 'a change is as good as a rest', 'enough is as good as a feast'. I can't think of another that specifically draws a distinction between two things like 'as different as chalk and cheese'. Why the need to make the distinction? After all, chalk and cheese aren't similar enough for anyone to confuse them.
The earliest citation of the phrase, in John Gower's Confessio Amantis, 1390 does suggest some dodgy dealing, in which cheese is replaced with chalk:
The English language is packed full of phrases that contain pairs of rhyming or alliterating words - often for no better reason that whoever coined liked the sound of them. For example, hocus-pocus, the bee's knees riff-raff etc, It doesn't seem likely that the naming of chalk and cheese as items that are specifically different has any more behind it than the alliteration of the two words.