I think that many drivers/cyclists are confused by hand signals because so few cyclists bother to use them. If cyclists would use proper signals on a regular basis, drivers and riders would know the proper signals. One very good reason for the left arm indicating a right turn is the natural tendency to lean right when turning right. Extending your right arm while trying to lean into your turn is not only awkward but possibly dangerous as well. I usually use the right arm for the right turn but there are times when that left arm indicating a right turn is quite useful.
Maybe it's an age thing but I find it surprising that ppl who cycle on a regular basis would not know the four hand signals.