Sim, eles estão

Citando o Multiclasse chapter of Character Advancement rules:

Instead of gaining the abilities granted by the next level in your character’s current class, he can instead gain the 1st-level abilities of a new class, adding all of those abilities to his existing ones.

...

He adds all of the hit points, base attack bonuses, and saving throw bonuses from a 1st-level wizard on top of those gained from being a 5th-level fighter.

Multiclassing is considered an advanced strategy for experienced players. Forfeiting higher-level features of your class is realmente not to be underestimated: if done wrong, it makes your character substantially weaker. If it didn't add BAB, saves and hit dice of all your classes together, it would probably just be an inferior strategy instead of an advanced one.

Anyway, the part that might have confused you was probably that you can get some statistics higher than a character would have if advancing normally. Example: a level 2 Rogue Desencadeado has the following saves: Fort +0, Ref +3, Will +0. A level 2 character whose levels are Monge desencadeado 1/Unchained Rogue 1 will have substantially better saves: Fort +2, Ref +5, Will +0.

On the other hand, poor saves only increase once every three levels, so such a multiclasser wouldn't get their Will save increased at character level 3.

Este é apenas um of the reasons why multiclassing is considered advanced and not recommended for new players unless used in pre-made builds -- in which case it is actually not a new player who multiclasses, but rather the person creating the build.