Why does the game have two things happen at the same time?

To answer your question, a failure to think these things through, plan them out, and properly test the results. Writing game code is complex and there will always be things you forget or don't think of. Thus the rule, at least when I did stuff for BioWare, of test, test, test, and test some more.

This is not new to TS4. Back in TS2, the 'welcome wagon' three Sims from the nearest houses would come to meet your Sim. But if you had NL installed sometime within the day, a NL townie would call to invite your Sim downtown. You had to choose to go or stay at home. (You had guests, but meeting Mr. Big and or the Count vampire or someone was not easy) so you might run off to go with the downtown Townie.

I can't exactly remember but some opportunities in TS3 would pop up just when I had sent my doctor out to practice medicine on the streets, or at community lots, lol.

I think Maxis just has bad timing.

In TS4, my dishwasher would be in bed (got home very late after work) and get a call in the middle of the night to go somewhere with a random townie. Did they ever tweak those middle of the night invites? I doubt it.

As in they forget to engage brain before putting code into gear. On the positive side, on occasion in TS2 the invite to downtown would come from another villager, with more villagers as part of the group. Great chance to meet 5 more neighbors on the rare occasions it happens.