Yes, the logic behind it is that you pick the class you are advancing before you pick your skills. Knowing the logic will help you remember the rule.
1. How can you know how many skill points you are going to get until you know your class that you are advancing in?
2. How can you know which skills will be class/cross-class until you pick a class.
Anyway, in general just remember you pick class before skills. Thus, as you level up from 4th to 5th you look at your sheet and see that Skill X has 7 ranks in it. When you are picking a class and you see that PrC that says 8 ranks of Skill X you can't pick it yet. However, as you are leveling from 5th to 6th and you look at your sheet in order to pick a class and you see Skill X has 8 ranks now ... well, there you go!