When I play as my shielder I notice it seems that nearly any weapon will swing through a nice solid block (most often with 2h and poles, less often with 1h). If polearms do it more often, I have yet to notice, but when playing my polearmer and I seemingly swing through a shield it is usually a right-to-left swing where the head of my axe arguably swings around the side of the shield, due to the animation. It could be that that is the reason for it, which is definitely unfair to the shielder that anyone with a polearm standing off to the side can get around them, but is most likely unavoidable due to the ancient warband engine, unfortunately.