Don't forget some CFPs take into account CAT or the lowest
expected statistical level for that sector (eg lowest level that
can be expected FL280 and the CFP reflects that level's fuel
requirement). The box's optimum for same sector might be
FL340 so you take 280 fuel but you're ATC-planned at 340.
Talk to the dispatcher whether he took into account any SEV
CAT levels if the CFP level seems unusually low. Sometimes
the CFP can be out depending on the time of compilation and
the CAT may be simply moderate by then but sometimes the
reverse can happen.