how on earth can an amazon parcel delayed by 5 days?

heya all, just wondering if anyone can provide insight because this one makes no sense whatsoever to me.

So I ordered something from amazon and despite having prime, they decided to send it through royal mail. weird, but okay. I've noticed they tend to use RM for smaller items, but this thing isn't that small. Weird- but whatever, it's coming.

Delivery day was supposed to be today, so I check the tracking info on my amazon app. It's left the amazon depot. It's with the courier (RM). It's at their depot, but it's not marked as out for delivery. But it's allegedly "arriving today" (spoiler alert: It's 4pm at this point. It's not coming today.) and I decide to cut my losses and ask someone to wait for it for me tomorrow.

I get an update. "Now expected by 6 October. We're sorry for the delay. If you have not recieved your package by 6 October, you can come back here the next day for a refund."

Other than being (slightly) miffed about this bc I needed the item for going abroad on the 4th and there will be no-one at my address on the 6th, there's nothing I can do. Okay. Fine. Whatever. RM (in my area- I've heard it's great elsewhere!) is atrocious, I should have expected it all to go to hell.

But because I'm trying to understand this, like, it's legitimately confused me: if I order the same item again on Amazon, (I'm not doing this) I can get the item "tomorrow". So why 5 days to get from a local sorting office to my address?

If it's really at the royal mail depot, that's less than a quarter of a mile from my address. Why can't they just put it in for tomorrow's deliveries? Is it not really at the depot? Has it been lost and RM are attempting to find it and have 5 days as a blanket grace period to refind it? Has it been stolen? Has it fallen off the back of a lorry? Am I going to get a package that looks like it's been jumped on?

I'm full of questions. I'm well aware that having answers (or best guesses from experts for the liklihood of Whatever having happened) isn't going to make my item arrive, but I'm genuinely curious.