While the medieval mind was attuned to all sorts of symbolism & typology, perhaps much marginalia was intended and received simply as amusing entertainment. In some ways they may be akin to the captionless little black & white cartoons that were used as space-fillers in 1950s-60s newspapers and magazines, with the cartoons generally having little relationship to the subject matter of the surrounding text.The adversaries of the snail usually seem to be armoured-a visual pun on the fact that the snail is also armoured and the whole made more ludicrous by the unequal nature of the combat (a bit like Python's Sir Robin almost battling the Giant Chicken of Bristol). I recall similarly-themed cartoons and T shirts in the 1970s where a tortoise was attempting to mate with an army helmet.