For example, I'm taking issue with all BMW's costing $17,800 over 10 years on average. Yet, the BMW model with the highest maintenance cost, the 328i, only costs $15,600 over 10 years.
Edit: Tried, quite unsuccessfully, to improve clarity
It's because the brand estimates are not based on actual costs, but only on oil changes:
“In order to estimate annual maintenance costs, we found the amount spent on every two oil changes (as oil changes are generally done every six months).”
Yet the actual oil change intervals for at least BMW, Mercedes, Audi, and Volvo are between once a year and once every two years, or after between 7500 and 15000 miles, depending on the brand and model.
[0] For Americans, where Manual Transmission is much like the Big Foot. You hear about it, but nobody's actually seen it, this is a part of a Manual Transmission car that wears out over time and is guaranteed to require repair.
Even mention ignition coils around some of them and there will be much cursing.
Maybe I got lucky, but you rarely hear people's good experiences, and you often hear the bad ones.
The point is parts for imports here in the states are generally much higher.
If you have two sets of numbers, S and T, then I'm quite confident MEAN(S ∪ T) must fall between MIN(MEAN(S), MEAN(T)) and MAX(MEAN(S), MEAN(T)) inclusive.
In this case, the costs of maintenance for a given vehicle model are the sets and, so, the cost for a brand as a whole would be (S ∪ T ∪ V...).