What term describes the discrepancy in reported intention to vote and actual voting behaviour?
There are many reasons why stated intention can differ from behaviour (e.g., see Theory of Planned Behaviour). Social desirability is one possibility. However, there are many other factors. A person may not know who they are going to vote for or may not have decided. Factors may arise between the point in time when asked and when voting occurs that change the person's voting plan.

Furthermore, there are many potential limitations to polls that can lead their estimates of population voting intention to be inaccurate, and even biased. There is basic sampling error that arises whenever you estimate a population property from a finite sample. There are also a wide range of issues related to getting a sample representative of the voting population (see sampling bias).

Furthermore, polls form part of the information available in betting markets. Thus, in betting markets, people are able to use an ensemble of information including multiple polls, various historical trends regarding how polls change over time, various adjustment to poll predictions that might remove perceived biases and so on.