I think you're missing the point. There's not really a "30k service" per se - at least not for every car... the third service is only due at exactly 30,000 miles if the second service was done at 20,000 miles.But that would be 1000 miles outside of the 1000 miles I'd been quoted over the 30k would it not?
If the first service was done at 10,500 miles, the second wouldn't be due until 20,500 miles (10,500 + 10,000). If you used the 1,000 miles contingency and got the service done at 21,500 miles, then the third service wouldn't be due until 31,500 miles (i.e. 21,500 + 10,000). If you again used the 1,000 contingency and got it done at 32,500 miles, then the "40k service" wouldn't actually be due until 42,500 miles... all within the stipulated rules for service interval.
Of course this assumes that you do a normal mileage of >10,000 per year.
If you only do 3,000 miles per year then your fourth service would be due at about 12k miles, not 40k miles!
Hope that makes sense?