and as to gas utility, I can't speak for tricity and water, one buys on a whole range of factors depending on volume needed by the client and the seasonality of that profile (depending on geography, temperature and load factors). So think of the supply side as a series of longterm contracts, say 3 years - where price may move marginally, medium term of 1 year and then the supply profile topped off by spot deals, and sometimes paper is used to hedge pricing in a volatile market. When you 'make your measurements' a supplier like SEE or even BT will have very different profiles and load - a lot of the comparisons are made by looking at these short term deals but then not taking into the longer term contracts that are less price flexible.
In the oil markets the same goes on - out of roughly 100 mln barrels a day globally, only 10 to 15 million is traded per day, though those cargoes/lots can go through traders hands, six or seven or more times a day. Most oil has contract prices that may contain a percentage of flexibility tied to Platts (the major benchmark for trading pricing), or things like ceilings and floors that keep the price constant within a tunnel, but then can carry some deflation or inflation if the markets move beyond the lines.