I would guess the entire length of the steel tapers toward the edge, that is to say the steel is thicker toward the handle and narrows at a light angle up until the cutting edge bevel. The side bevels would appear to follow this taper. This is supposition entirely, since I cannot see a side view of these. If that is so, the bevel would be helpful, and the shape of the steel would also make the chisels incredibly strong and super efficient at transferring force down into the tip, compared to a flat piece of steel.

This is all conjecture -I'm just flat out guessing-