Hello,
It is mainly due because you probably didn't enter any routing to your altn + approach in the primary F-PLN. So the distance taken by FMGC is direct dest to altn.
On the other hand, it is quite probable that your company provides you with a flightplan which plans the altn in a certain way (direct appr or full ifr appr, routing to altn longest or...).
Best is to check with your flightplanning department.
CI=0 means max range which is logic for an altn fuel calculation. I suppose your flt plan altn fuel is also max range (but check...).
Of course for not too far altns CI will not make that much of a difference...
I would use personnaly (see your sop's...) the FMGC to xcheck your company fltplan.
So I would force the required planning fuel to your altn in FMGC = flt plan
This will allow you to xcheck that you have enough for dispatching the aircraft (xtra fuel/time) ! (wind in there/step alts etc...to be 100% correct or at least be on the safe side)
In flight you can eventually remove this forced planning altn fuel after having entered the full route to your altn+full approach. Then you'll get the real fuel consumption along your routing and a new min diverting fuel.
FMGC doesn't take abnormal flaps/slats/gear... affecting your FF into account, be aware.
Be aware that the route you enter will maybe not be the one you'll receive, possible holding at your altn, which approach etc etc..... The idea is to land with final reserve at least at your altn this by operating the aircraft with anticipation.
Enjoy.