Those are actually 4 X 8 bunks meant to be mounted vertically. Solid and quite heavy.
They have an angle on the top to accomodate for the hull angle.
I know the bunks you are talking about which are sideways 2X6 or so.
Very common to see trailers with these bunks down here for some reason. I guess it might have to do with our marinas and ramp heights.
This site shows the two different models:
http://www.continentaltrailers.com/