cwatters - there are numerous threads on PPRune about fuel reserves/diversions/short of fuel etc - search will locate. There are many reasons why I may not arrive in the destination area with enough fuel to continue to destination at the expected landing time, even if I have taken the company 'minimum'/'required' fuel, added on some for mum or even had shortcuts and more favourable winds. Too many variables to list.
Are you familiar with the term 'contingency fuel'? If not, that would be a good place to start.