For the first 500 miles after the service the indicator reads ---, then over those 500 miles it works out how many miles till you next service.
If you do one 500 mile trip at 60mph it will probably say 25,000 miles, however, if you do those 500 miles all as short, cold start or city driving it may say 12,000 miles.
You may find that yours first read 14400 miles and since then you have been do the sort of milage that gives you a higher service gaps, meaning you are waiting for it to drop down below 14400, this may not happen for another couple of thousand miles. Take it out and thrash it for the next 500 miles and I bet it soonn drops!
My first 500 miles were all short runs and it said 12300 when it came up, since than I have done 2000 miles of steadyish driving and it still says 12000 miles.
There was a pdf somewhere with how the service computer works it all out, on a diesel it starts at 24,000 and then something like 4 miles is knocked off for every mile driven while the engine is not at optimum temperature, 3 miles knocked off for every mile in traffic, 2 miles knocked off for every mile done over 3500rpm and then only 1/2 mile knocked off for every mile travelled under 2000rpm.
That is not exactly right but it was along those lines and it became easy to see how you can go from 24k miles between services to only 15k miles between services.
