If you are having any profiles like Persistence and/or OneConnect which directly affects the load distrubution, it may result into uneven traffic distrubution across pool members. This could be one of the reason.
Another reason is, that Round Robin is a static method and doesn't care about any unequal load over time. It might be the case that requests to server A require more time (either to more complexity or because server A has less performance than server B) and sessions will be closed later.
That's why I would recommend to use a dynamic method like Least Connections to handle such scenarios much better.