In a free market there will be fluctuations. The utilities enter long term contracts for the base load and then fill the remainder of what they need with short term and on the spot bidding.

Fuel cost fluctuations, mostly coal here in the Philippines, are passed on to the utility. If one power plant shuts down for maintenance or repair then the excess power required from the other plants is more expensive. Older contracts expire and newer contracts are more expensive. (Labor increases, etc.)