I have two theories, regarding resource precision and individual vs. social value.

The former suggests that time is a resource requiring no greater precision than others we employ. If a recipe calls for 200 mL of white wine, for example, I'll just chuck a glass in, or glug some in from the bottle, by eye. It doesn't really matter if it's actually 170 or 250 mL (spoiler: it's going to be the latter).

And so 7 pm needn't be '7 pm'. 7.20 actually performs the same practical function of 'this evening not before 7 but not after 8'. But we have to put a number on it for it to make sense. The chef doesn't really expect exactly 200 mL of wine to be used, but we (in Northern Europe) do expect arrival at 7.00.

And then there's the idea of 'selfishness' associated with lateness, which may also have to do with the more individualistic mindset of Northern Europeans.

Let's say you arrive 30 minutes late, because you were chatting with a lonely elderly neighbour (this is exactly the kind of thing my wife would do - it's how her priorities work).

I 'lost' 30 minutes kicking my heels at the bar. Though in fact was able to entertain myself with a glass of beer, some peanuts or olives, and a couple of articles on Medium (other platforms also available).

But our broader society received value from the reallocation of those 30 minutes. Maybe, as with your mum emptying the washing machine, both functions could have been fulfilled with the same resources, through better-planned allocation. But maybe not - sometimes things just happen, and have a knock-on effect.