If I am a 5th level fighter and reach 15000 XP, I can not take a level of barbarian, why? Because I am of lawful alignment. I don't meet the requirements for the class
If I am a 4th level fighter and reach 10000xp, I can't take a level in Eye of Gruumsh. I don't meet the Base Attack Bonus requirement.
if I am a 6th level Druid, have great fortitude, reach 21000xp, but only have 3 ranks of swiw, I can't become an Ooze master, I don't meet the criteria of 4 ranks of swim, and can not take a level of the Ooze.