both of those examples happen the way they did because guile’s s.lk has an active hitbox out in front of his hittable hitbox, where as blanka doesn’t, therefor guile outprioritizes blanka because his move is touching a hittable hitbox while blanka is out of range. same thing with the ken situation, ken only has an active hitbox at the front of his mp, no hittable box there so when blanka runs into that he can’t cause damage because his active hit box is not touch ken’s hittable hitbox.
Priority is a lie.
You could study frame data, and learn all the numbers, but what it comes down to is matchup experience. Just play. you’re not going to get better unless you play.
Let me give an example I’m comfortable with.
I’m playing Gouken, and my opponent is playing Sagat. Keep in mind this situation actually happened tonight.
When he jumps in (he’s not a very good Sagat, mind you), and attempts to hit me with his medium punch elbow, I want to crouch and knock him out of the air with medium kick.
If you look at Gouken’s cr. medium kick, he gets down even further than he already is, and sticks his foot out and up at a 45° angle. This shrinks his hittable boxes, and creates a hitting box that’s almost as tall, if not taller, than his hittable boxes.
Sagat’s ‘People’s Elbow’ j. medium punch has a fairly thick hittable box, but his hitting box extends quite a bit past his elbow horizontally.
HOWEVER, since the hitting box of Gouken’s crouching medium kick almost always gets to the hittable boxes in Sagat’s j. medium punch first, the medium kick is stated to have priority. There will sometimes be trades, but in 90+% of the cases, as long as your timing and spacing is decent, you’ll win this situation (score some damage, interrupt their plan, and get some time to set up an offense).
TL;DR: ‘Priority’ is just a way to say a lot in a short term.