Constant adjustment of prices isn't a practice that has been adopted, per se. Gasoline is made from oil which is one of the key international trade commodities. One of the main concepts in economics is supply and demand which affects prices. If there is high demand, and low supply, prices will increases. Low demand, high supply, prices decrease. Its simply a matter of how much the commodity is wanted versus how much of it is available.

In the case of oil, there are lots of factors affecting this all the time. Demand is always high of course because amongst a myriad of applications, you need oil to produce plastics, and for gasoline.

Most of the worlds oil comes from the OPEC nations in the middle east, if they artificially limit supply then the price of gasoline will go up. Typically in the past they have manipulated supply to keep the cost of oil over a certain level but in the last year and a half this trend has reversed with OPEC keeping supply high, driving prices down.

Other factors include international sanctions. For example, international sanctions on Iran were recently dropped as they renounced their plans to enrich uranium. This meant they were able to export their oil in far greater quantities, thus driving the price of gasoline down further.

The cost of oil as a commodity on the international market changes daily as a result of these, and other factors, and thus, the cost of gaoline is constantly adjusting.