I just had a similar issue but a bit different.
I had my car serviced at the normal interval but at the 11 month and with 10k on the clock in April 17. Since April, I have done another 7k in in 6 months.
When I alerted the service dealer, he indicated that it means that the oil additives have about 10% left and when the "change oil" comes on it is time to change the oil.
Like others I questioned the time and Ks travelled and he advised this could because of the type of driving eg in traffic, stop start, using the performance mode, track work etc or simply they forgot to reset it.
I explained to him that being retired, I avoid peak hour traffic, don't use performance mode or go to the track and tended to think his last suggestion may be more relevant. However, it may still come back to the way I drive in that if the "change oil" sign comes up around the 20k mark it could well indicate my driving style rather than the time factor.
Interestingly, the service dealer indicated that the Ford Escape has the same system and they had similar enquiries (but not on the Mustang, I was apparently, the first) and requested feedback on how I go with waiting until the "change oil" light came on.
I will inform him and the forum in due course.
