I don't understand - why would they continually vote against their belief system?
If I may, please allow me to explain. There are two basic types of personalities - thinking-centered and feeling/intuition centered. Many people are basically thinking-centered in their working lives and in matters of day-to-day practicality. But in peripheral matters where their own immediate best interests are not at issue, they somehow dissociate and allow their feeling/intuition-centered cognition to dominate.
For thinking-centered people to hold a strong opinion, they require some sort of strong rational argument with facts, logic, figures, reason etc.
For feeling/intuition-centered personalities, all they need to hold a strong opinion is a strong feeling or intuition. For such people rational information such as facts, figures, logic, reason are of secondary (or lower) priority in their consideration.
Whereas thinking-centered people often use deductive reasoning to arrive at conclusions, feeling/intuition-centered people use what CG Jung called "impressions" which are a random mélange of various bits of rational information (often selected inductively, not deductively that is "cherry-picked" to support a pre-decided conclusion) combined with intuitive snipets of past experiences which relate to the topic.
Sometimes people who are required to be hyper-rational in their professional lives seem to fall back into hyper emotional/intuitive reasoning in matters like politics or other things - almost as if they are compensating in their consciousness for being obliged to be so strongly rational in their work.
Many times, however, it is just a matter of habit or convenience. They have coworker, bosses or other close relations who are leftists and it's more convenient or fun for them to adopt leftist attitudes than conservative ones. Many far leftists I have encountered are not very serious-minded about their political views but maintain them much the way a hobbyist maintains a collection of comic books, coins or stamps. They never invest much thought or effort into the endeavor and seem to resent people who respond to their casual attitude as if it is in any way irrational or inappropriate.
Some people consider intensity on the topic of politics or ideology a sign of inferiority or being a "bad person" in some way. I don't claim to fully understand such attitudes, I just observe them sometimes and take note.
For the most part, I think political conservatives tend to be a little more (in some cases a lot more) serious minded about life than leftists - who often view life as being largely a matter of convenience and fun, not serious, thoughtful consideration or consistent moral doctrine.
