Harrison Hickman, a pollster and political analyst, has this interesting article about the three Classes of U.S. Senate seats. This year, the Class II seats are up. The article explains the statistical anomaly that the Class II seats are quite different from the Class I and the Class III seats. 2014 is a year in which the Class II seats are up.
Every southern state has a Class II seat, except for Florida. By contrast, only five southern states have a Class I seat.
The population of the states with Class II seats is considerably lower than the states with Class I and Class III seats. This is all just a result of a random process.
The U.S. Constitution went into effect in 1789, and said U.S. Senators would have six year terms. Congress then held a lottery to determine which of the U.S. Senate seats fell into each of the three classes. The purpose was to determine when each seat would be up for a new election. One-third of the Senators elected initially were assigned to Class I, and they had to run again in 1790. The Class II seats were up in 1792, and the Class III seats were up in 1794. The pattern has continued to this day. When new states were admitted to the Union, their two Senate seats were assigned to one of the particular classes, so as to keep the number of seats in each Class as equal as possible. Today, there are 33 Class I seats, 33 Class II seats, and 34 Class III seats.
Particular seats never change their Class. If a Senator is elected in 2010, and resigns in 2011, a special election is held for that seat in 2012. But since that is a Class III seat, it then has another (regular) election in 2016.