Ah, I misinterpreted. so if the Castillians captured a chunk of territory in one go, whether it was a segment of one Taifa or multiple Taifas, they'd just name it one Kingdom? That makes sense. I'm always surprised Extremadura wasn't the Kingdom of New Leon or something like that. Castile and Toledo seem to have more in common than (old) Leon and Extremadura.
It's a bit more complicated.
Depending on how were the relation between the taifa and Castile, how it was conquered, what was its strategical importance, its prestige, who were the settlers coming there, it could be a kingdom.
Generally so, the border correspond to taifas but do not match exactly these. Don't forget that often the borders presented on maps for these kingdoms are XVI/XVII ones and that could differ from actual XIII kingdoms.
Actually, no. Leon and Castile had a lot on common, culturally, historically.
You don't have an opposition between Leon and Castile for itself, just the usual local rivalities that you could found between Castile and Jaen let's say.
For Estramadure, it's because the name is old and widely used (On the other side of the Douro). It's for the same reason that "New Philippines" was quickly renamed Tejas by everyone.
But actually, even in Mexico you didn't had a "Kingdom of New Leon" but a "New Kingdom of Leon".