My understanding is that the car has several types of service it wants, oil, air con, brakes, brake fluid, coolant etc.
They all operate within certain timescales and I think the oil one has some wiggle room in it depending on how the car is driven (hence why I get longer as all I do is trundle up and down motorways, I've done almost 20k in this car this year).
Obviously all these end up happening at different times, so my brake service may well not coincide with my oil service but when it goes in for the brakes they do just that, not the oil.
Then with the oil, every 3rd service they do a few extra filters but again I think every time they do an oil service they chuck out the old oil and filter and change it, it's just every 3rd one they find a reason to change some other filters and charge you another £200 on top of the normal fee.
Happy to be proven wrong but thats broadly what I think happens, hence why some people are saying they don't change the oil every service, that'll be because not every service has anything to do with the engine at all.
What I can say is the service countdown thing does not run at 1 mile down for every mile I drive, I'd guess I get something like 1.5, so when I was getting close to being ready for the service it was saying I had 1000 miles left for ages, some of that will be the fact it doesn't count down in single digits but I'm pretty sure I'd have done something like 500 miles before it even budged off 1000.