Palisades are built in smaller segments than stone walls. When a ram attacks a palisade, it attacks the edge of the palisade, the center of the palisade, and the other edge of the palisade. Each part of the palisade has 700 health, so an un-upgraded ram, dealing 150 crush damage, will need at least 5 hits to destroy each part of the palisade. Having destroyed a section, a tiny bottleneck of soldiers can then follow the ram through, but they are especially vulnerable to the defending army since they must squeeze through a small space. (Another trouble for the invader is that soldiers (especially ranged soldiers) get distracted fighting the palisade and do not immediately stop when enemy soldiers come into range)
Stone walls are built in larger sections and of course have more health. But once the main section is destroyed there is a large gap for an army to pass through.
I think palisades are too strong versus siege compared to stone walls and the main issue is that they have smaller sections that each need to be destroyed. I propose that the length of the middle section of palisade walls be extended to the same length as the stone walls.