Regulations don't seem to allow for additional contingencies. Diverting to an alternate is often associated with additional weather diversions and ATC sorting out multiple diversions to same alternate. Each of these can add considerably to the alternate burn.
Sometimes alternate itself can have good weather, but many aircraft will need to backtrack to exit, meaning 4 nm separation on final is insufficient causing additional high burn at low level. A go around due an unexpected missed exit by preceeding aircraft when already at 30 minutes would be no fun, especially if other aircraft are also claiming very low fuel.
Any captain who always accepts the company's computer flight plan minimum fuel is a idiot. Company pressure to accept is intolerable and pilots should, as required, adjust fuel to their own requirements. Resist pressure, regardless of any conception of company consequences.