Artificial shortages are a joke. Basically before the spring and summer driving seasons, oil companys perform maintenance on the refineries that last about a month. They pretty much take them apart and put them back together to make sure they are working correctly. I can't get a straight answer from my bro-in-law as to why they don't perform TPM maintenace throughout the year instead of this, but this causes a drop in production that you see at the pump. These procedures are why they get the large subsidies from the government, although the subsidies are supposed to prevent them from having to raise prices because of a production decrease. It's a vicious circle that will continue until gasoline isn't the #1 energy need.
The reason for doing the maintenance all at one time is because refining is a continuous flow process. You take one distillation column or one reactor out of the picture and it can mess up the flow for an entire refinery. It is also impossible to do much of the needed maintenance, such as checking the trays in a column, while the column is in operation. It is much easier to shut down the whole plant, bring in a whole lot of people and get everything done at once than it is to try to figure out what to do with all the stuff that suddenly has no place to go because the distillation it is supposed to go to has been disconnected.