Mega Man Zero 2

I read that you should have a high rank (A or S) that means completing the primary goal of a mission, killing a given minimum of enemies depending on the mission, and completing a level in a given time limit, no retries, no elves etc...

Here's the problem, the snake boss gave me its ex skill eventhough I had a rank of D and received damage both from the enemies and the boss, I dealt to it high damage attack (the triple sword combo) two times or so, so I'm doubting that's what made it do its ex attack (lasers fired from the big snake in a stairs formation when the spring thing is going down them) which means I had a high rank (???) but received a D in the rating chart (...wtf)

Now that I played the power room level, defeated the phoenix boss with no damage, minimum time (2'29), minimum # of enemies defeated (55) I received a total of 100 points, and ranking of A, but no ex skill, same with the other ice and thunder bosses

I saw some dudes omitting the missions when possible (like destroying the cooling system of the ice level), completing the levels less faster than me, and inflicting elemental damage to bosses occasionally, and obviously getting the exskill

So the question is, how do you exactly get a boss to give its ex skill
or is there some serious glitch with the european game