Actual diversions can be very different to fuel planned diversions. Very often nearer alternates are available, but ATC won't allow them because of space considerations. In a 747 holding at LHR once, instead of being allowed to divert to MAN, the nearest we could get clearance for was PIK, on lowish fuel. MAN was 'reserved for narrowbodies', BHX was fogged. So 32 widebodies ended up in PIK, some desperately low on fuel. That's why pilots like lots of extra when there's fog about!