Maybe the Royal Mail delivery times depend on the trunk route taken, and whether the supplier delivers to their local post office, or RM collect direct from the supplier?
I have been using one particular supplier for many years for parts for my business. Delivery times used to be quite hit and miss.
A couple of years ago they switched to just using RM, Parcelforce (very heavy items) and UK Mail (for items that RM and Pf won't carry - batteries, cleaning fluids, etc).
Since this strategy was adopted, deliveries have been like clockwork (pun intended - they are a clock & watch parts supplier). I place an order nearly every week, choosing RM tracked 48hour as it's the cheapest option. Without fail, the order will arrive 2 days later, as long as I order before 3pm (before lunchtime if I place the order following a bank holiday weekend). Perfect service.
It's a big company, many hundreds of orders a day. Everything is tracked as it is picked, all on CCTV. Only once has an item been missing from an order. Quick email to the company, CCTV checked, error spotted, part shipped that day.
Orders are logged on the RM tracking system, collected the next day, shipped 400-odd miles, delivered to me by lunchtime the next day. They charge £3.45 for up to 2 kg. I really couldn't ask for a better service, which is why I gave them £5k business last year, and it's part of the reason why they have grown to be the biggest supplier in the country over the last 10 years or so.
I don't expect that kind of service from a smaller company, but some idea of how long it will take, and consistency week to week, seems reasonable. I have a lot more choice of supplier for hobby materials than I do for my business parts, so I'd expect suppliers to think "if I do a good job, I'll grow my business by keeping existing customers and gaining new ones". Maybe hobby-trade customers are just too laid back about it, so businesses don't have to try too hard?