Units don’t teach what the titles say

Hi, I don’t know if this has been addressed here before, but I wanted to ask, if you guys have the same issue or what the reason for this problem is.
I’m doing Korean lessons every now and then, I’m not an avid learner by any means, but I’ve already got some knowledge on grammar from going to Korean classes, so I know few things.
Currently I’m on section 2 unit 15 and I’ve noticed that some grammar exercises aren’t about the grammar topic that’s in the title at all. In my case, apparently I was supposed to learn “basic past concepts” for a few units already, but none of them actually had any phrases teaching past concepts.
Have you guys dealt with something similar as well and do you know why it’s like that?