Bicycle helmets are designed to protect against a single, large impact (like a motorcycle helmet). Which is a different task than protecting against a smaller impact (like your incidents) or repeated smaller impacts (like a football helmet).
To protect against concussion from a small impact, you'd want a helmet with softer cushioning. But to protect against skull fracture and other major injury in a large impact, that cushioning would be too soft, and would compress too easily (like when car shocks bottoms out); you actually want harder cushioning - which doesn't protect as well against the small impact . . .
Well, we can't have it all, considering the helmet is also supposed to be light, aero, compact, and inexpensive. Plus most helmet test standards were developed before the current concern over concussion in small impacts. They were more concerned with your skull not being split open or crushed in a 30 mph crash, than about avoiding a concussion in a zero mph fall.
There's a good article on this in the June 2013 Bicycling magazine. There are two different systems being used in helmets to try and reduce the forces that cause concussions. One is the MIPS system. Scott helmets was an example given of this.
http://www.scott-sports.com/global/e...3378F7A058E1EE