Generally, forum moderators are displayed as “staff”, but in fact most (before recently, even all) forum moderators are voluntary player mods and not actual flaregames employees.
As such, we generally have no access to game internals or secret internal informations, but we usually have quite an amount of experience with this forum’s content, with the game mechanics (being active, veteran players, same as many other forum members) and with “hidden details” in the game. Thus we can mostly speak of our experience, not of the game’s source code.
Concerning the cannons, they seem to have a hidden resistance to piercing damage. Similar, mummies seem to have a hidden weakness to blunt damage, spells are more effective against the castle gate than troops (with same damage types), frosters have (despite stated otherwise ingame) no general blunt resistance, etc etc… try using the search feature to find some older, more extensive topics about the “chaos” of resistances and weaknesses.
Personally, I’m proposing showing all those things ingame to give players correct information and not partial, wrong information, but so far flaregames didn’t do so nor comment on that issue. Thus, it’s hard to say what’s going on *exactly* with resistances/weaknesses behind the scenes.
And we can only speculate it’s for game balancing reasons that those things exist, and for laziness or technical difficulties that they aren’t shown in the game properly.