I have noticed that too, without looking in the data though.
For me as a user it is quite frustrating, because when I invest some time into answering, my previous experience on this site and with gamified services tells me I will get some positive reinforcement for my efforts. When it's not happening, the absence of positive reinforcement is not neutral, it turns into a negative reinforcement instead.
One of the strategies to fix this would be for moderators to remind OPs that they are to choose their preferred answer, and not just use the information that was shared with them.
Another one: upvoting all people who answered your question (unless you profoundly disagree with the answer or find it irrelevant) to me sounds as a good form. I think it should be propagated as one. Since these people have invested their time into answering, thus resolving your issue and helping you, it is only fair that you upvote them as a thanks for their attempt.