With blues, both major and minor pentatonics can be and are used all the time. Even better, with blues (and it makes sense!) would be to use major and minor blues scale notes. I'd go even further and advocate using chromatics too, mostly as passing notes, so basically, any notes, almost anywhere, if they sound good, will be the order of the day.
That said, here, there's partly the circle of fourths going on. Yes, it's in E♭, but moves to C, then up 4 to F, then up 4 to B♭, then E♭, and goes round again. As it's doing this, a lot of players would 'play the changes', and use each chord to provide certain chord tone notes that fit more exactly over each chord.
A to E♭ is a tritone interval, and that's one way to explain it theoretically. Another could be that the A is a semitone above the A♭ just before it. It's just an effective change - I last played it in 'Blues in the Night', but it went back to A♭ straight after.
Often there isn't a straight explainable reason which could justify 'theory', but as we often say 'if it sounds good, that's good enough'!