This has something to do with old-style printing presses, but quite what the issue is regarding your question on punctuation I've no idea, other than it might have something to do with the physical sizes of the lead letters (I'll have to leave that bit to the experts here!).
Type metal is a low-melting alloy made by adding molten antimony to lead containing a little tin. This metal cools quickly, pours easily, and can be remelted without too much effort (so old type can be recycled and recast into new letters). From: http://work.colum.edu/~matma/letterpress/lptype.html