Translation:Some of you still do not understand this.
I was mystified by this, too. So I tried a few "some of (you, them, us) don't understand" translations on Google, and they all came out with the verb in the third person plural.
Granted, Google isn't exactly a foolproof source, but, given the consistency, this may be something that falls into the "that's just how we say it in Czech" category.
Theis explanation is correct, the subject is někteří=some, it always has third person verbs, no matter from whom are the some. z nás, z vás, z těch lidí, z této školy,...
I don't think English is really different, you just don't distinguish the verb forms. Svrsheque's explanation is spot on.
If it doesn't seem strange to you, good. But I think it does seem strange to English speakers. You (pl.) don't understand = nechápete. An english speaker would ask why this should change if only some of you don't understand. Bonehead Bass is right (as usual)... It's one we just have to learn is different in czech