I saw this years ago and I still think it was primitive office humor. Snails ate delicious plants and there were probably monks waging a war against them. The incredulity of fighting so hard against an enemy so weak was funny.
This seems like a plausible explanation, but I'd maybe expect to see a few giant slugs and caterpillars - these are at least as damaging to crops as snails.
Im betting they got all done up in their armor and went to fight in muddy battlefields, and when they were not fighting the enemy they were dealing with snails crawling inside the armor and being all slimy and disgusting crawling between the knights legs and the armor.