Now there may be several answers to this one...
1. The company may have decided that there is a minimum of 200 kgs of taxi fuel for all there operations....
2. The company may have decided that even with the necessity of 100 kgs only, the amount of 200 kgs is a safer way to operate as this will cater for unforseen delays....
3. The captains have decided to always use 200 kgs as a minimum as this provides a few minutes "pocket fuel" for unforseen inflight delays....
4. It is somehow common sense and backed up by any FOM, if the captain decides for some reasonable extra fuel, if he sees a reason to do so....
5. Sometimes the second-in-command has a valid point suggesting some fuel for the family at home....
6. Experience of recent operations into a particular place may show the demand to cater for some additional fuel....
The list can be continued for may other reasons, the given ones here only show a few which come to mind on first thought...