Unforecast headwinds would be the obvious answer.
My company had a fuel incident enroute to Australia due to a pacific island ATC- par for the course- incompetent hold down causing excessive fuel burn. The pilots elected to drop into an enroute alternate to pick up fuel so as not to arrive in a major Australian city with less than the required extra hold fuel. Aussie ATC caused a stink..CASA passed it back to our home regulator which made a 5 minute call to me where I explained the situation, and got told by them it was a ‘storm in a teacup’ and CASA would be told to bugger off in the nicest possible way