This may occur where there are unforeseen additions to the original requirement, where tenders are received more than the available budget, where funding levels change etc.
Gall hyn ddigwydd o ganlyniad i ychwanegiadau annisgwyl i'r gofyniad gwreiddiol, lle derbynnir tendrau sy'n fwy na'r gyllideb sydd ar gael, lle mae lefelau cyllido yn newid ac ati.