Future plus present simple or present perfect

Hello,
I have these 2 sentences:

We will continue the meeting when he has recovered
They won't know anything about it until they have got here ( they book sais: until they get here)

I don't understand why, the book sais in time clause like (until, when, after...) we should use a present perfect, but in this second case a present simple is used.

thanks in advance.