My Spider jumping in the lake disagrees with your to-hit odds :).
Skill 3 + TMM 4 + Medium Range 2 + Partial Cover 2 = 11+. And if there is any water or woods on the map, it will be there.
Two Spiders fighting? Tear my eyes out. Since they'll both be jumping, they can't possibly hit each other at medium range/cover. And even at short range, they still need 11+. (Skill 3 + TMM 4 + Short Range 0 + Partial Cover/Woods 2 +Jumping 2 = 11+).
Skill definetely helps, but sometimes you don't get to choose skill (playing a scenario with prebuilt forces, or the scenario dictates lower skill levels). The game can't just fail because the battle is two regular skill light companies. That should be a fairly common occurence. Though a Spider is certainly up there as an extreme.