>Perhaps the use of both "single quote" and "apostrophe" ...

“a” ‘a’ a’ a'
I got the fist three variations shown above by using MS Word (it adjusted the marks in automatically, based on context) and the last by typing directly in this software. I would have thought that in a proper printing font the ‘ and ’ would be different but before word processors replaced typewriters the ' and ' would have come from the same key stroke on the keyboard as the apostrophe.

By the way, just to add more complication - my father calls them "lip marks"!

Foot Note: Mmmmm - That lost a little in translation, the edit post screen allows in " and " to be different when copied in from Word but it all gets "smoothed out" back to looking the same at the preview stage - I give up!