Its a chronic stupidity issue at dealerships & other oil-change places. My "free" changes at the dealership (Chevy) always ends up overfilled by about a quart. I check and remove some.
I concur with others above that say it is likely because of the slow drain time in many engines. Ecoboosts, GM's small turbo engine, and others, slow to drain, and the dealer tech doesn't have the patience to wait, and then they just put in the "spec" amount, overilling.
Its better to deliberately subtract 1 quart from the spec amount, put that much in, and then gradually top off over the next month or two, adding a small amount each week you check the level. Its like haircuts, only opposite, its easy to add some in, but a little more annoying to take some out.
For this Mini engine, the subject of this thread, it does have BMW's oil level sensor. The Owner's Manual oddly doesn't explicitly mention an over-fill sensor dashboard warning, only a low-oil warning. BMWs in the past were always capable of sensing over-fill so I'm not sure if this Mini is somehow diffferent. (??)