Results can appear to change if a voter changes his or her mind about which version they prefer.
Voters can change their minds and decide that they really love option 'b' more than option 'a.' They have the option to go back to the design and change their vote. Their most recent vote will override their previous vote, thus changing the results that you see.
In the end, this provides you with better feedback, as it's often the result of more careful deliberation and consideration on the part of the voter.