Technically if written like that all of the lower extensions would also be present - 7 9 and 11. Of course in practice that never happens, but it could lead to voicings that don’t quite capture how the chord was played.
Typical jazz nomenclature notes the 6th/13th interval as inherently major (Dorian), even when the chord is minor, so it should be b13.
I guess I learned it wrong then lol! Which doesn’t remotely surprise me. I’ve just synthesized that stuff from reading various published things. My education was classical and they don’t often use chords that adventurous. Plus, that was a lifetime ago.
Thanks for explanation! 
It’s just like how 2 out of 3 major chords in a key have a major 7th, and yet the designation of just 7 (like C7) implies it’s dominant. You have to add extra crap (maj7, or a triangle) even though technically it occurs more frequently. It’s the ii V superiority bias. lol
But you have figured bass! 
this is something i didn’t know - thanks. 