You guys do not get it because you are overly thinking about it. You are taking what the npc said too literally. Also the way you worded the child birth thing was pretty harsh. The term "at a time" is a vague phrase, with various meanings. It can be consinously or simultaniously, but it also mean per occurance.
I'll take the movie rental again. See you can rent 3 movies right after you finish the last three, or when ever you feel like it. You still can only rent 3 movies out at a time. Vaguely, that is saying whenever you want to rent out a movie, it is only 3.

Npc do not really state stuff based on earth time. They go by vague terms. If they did use eath time, it gets people more confused. Games do not show everything. I can name the countless quests in ffxi that do not stated the japanese 12am wait befor continuing on. Same for the same quest. It don;t tell you to wait 3 hours for the great katana.

Nor does the dark knight quest tell you to kill 100 mobs.