I believe it is 2 things. First, your LBS did not properly fit you, He might have assumed you were stronger/weaker then you actually are. Fit is not all about your dimensions.

Second is that you are locking your elbows when riding. When you ride your elbows should be between ~140- 90 degrees bent. Or you are just gripping way to hard. Try resting your hands on the hoods or the tops of the handlebars. You dont need a death grip all the time, only when you are sprinting really fast...

your fingers should never get numb on a bike, unless it is really really cold.