I just think that whichever parent doesn't do it, has no idea what is going on.
Take my sunday night for example.
DP's parents popped over, a last minute thing to look at our broken fence, normally we would be at home alone (either back from the in-laws or them having gone home) by about 4pm.
His parents left at 6pm.
I start peeling the spuds for dinner, and put some sausages in the oven. Need to go to the shop so I pop out, leaving the DCs with DP. I tell him dinner will be ready by the time I get back, and am held up, I expect him to have turned the dinner off. He hasn't, didn't realise apparently, partly my fault for not saying I suppose, but it was burnt.
It's 6:45, dinner time was 15 minutes ago, and dinner isn't ready. The conversation went something like this.
DP: Why are you so pissed off? It's only dinner. I'll put some more on. Go and sit down(trying to help)
ME: But that will take half an hour! I don't have half an hour!
DP:It doesn't matter if the children go to bed a little later. Dinner will be ready for 7:15.
ME: They don't just need to eat their dinner though, they need dinner, and a shower and the baby needs his shower, and to be fed, and all this needs to be done by 7:30.
DP: That's not a problem, I'll put some more dinner on, shower the children and you can go and sit down whilst I sort it. That way the children will have had a shower, can have dinner at 7pm and easily be in bed for 7:45. That's only 15 minutes late.
ME: No! DS2 has to go to bed at 7:30! Which means DS1 needs to be in bed by 7:15 so he is asleep and doesn't shout me when I put DS2 down. Then Dd goes at 7:45. This won't work now!
DP: Why does DS2 need to be in bed at 7:30? How about you put him to bed whilst I sort the other tow out, leave them and dinner to me, you do the baby.
ME: (getting really frustrated now) YOU DON'T GET IT DO YOU? DS2 HAS TO HAVE A SHOWER AT 7;15, THEN BE FED UNTIL 7;30ISH AND THEN GO TO BED, IF HE DOESN'T HE IS UP ALL BLOODY NIGHT, ALL I EXPECTED WAS THAT YOU WOULD REALISE I WAS TAKING LONGER AND CHECK THE GODDAMN DINNER. NOW THE WHOLE ROUTINE IS MESSED UP, I HAVE A ROUTINE FOR A REASON YOU KNOW.
DP: There's a routine?