In some cases, the demands for services over the year may not be as great as envisaged, or income streams may be higher than expected.
Mewn rhai achosion, efallai na fydd y galwadau am wasanaethau dros y flwyddyn mor fawr â'r hyn a ragwelir, neu gallai ffrydiau incwm fod yn uwch na'r disgwyl.