Strange behavior of the course testing system

countDifferentVars.hs

import Data.Set

countDifferentVars rules variables = 
    size $ Prelude.foldl (\acc (l,r) ->
        let sets = Data.Set.fold (\s sacc -> if (l `member` s) || (r `member` s) 
                                             then s:sacc else sacc) [] acc in
        case sets of [x,y] -> (x `union` y) `insert` (x `delete` (y `delete` acc))
                     _  -> acc) (fromList (Prelude.map singleton variables)) rules

countDifferentVars_with_unbound_var_test.hs

import Data.Set

countDifferentVars rules variables = 
    size $ Prelude.foldl (\acc (l,r) ->
        let sets = Data.Set.fold (\s sacc -> if (l `member` s) || (r `member` s) 
                                             then s:sacc else sacc) [] acc in
        case (sets,(l == r)) of ([x,y],_) -> (x `union` y) `insert` (x `delete` (y `delete` acc))
                                (_,True)  -> acc 
                                _ -> empty) (fromList (Prelude.map singleton variables)) rules

final.hs

import Data.Set

countDifferentVars rules variables = 
    fst $ Prelude.foldl (\(c,acc) (l,r) ->
        let sets = Data.Set.fold (\s (l_set, r_set) -> 
                let e = ((l `member` s),(r `member` s)) in
                case e of (True, False) -> (Just s, r_set)
                          (False, True) -> (l_set, Just s)  
                          _             -> (l_set, r_set)) (Nothing,Nothing) acc in  
        case sets of (Just x, Just y)  -> (c - 1,(x `union` y) `insert` (x `delete` (y `delete` acc)))
                     (Just x, Nothing) -> (c,(r `insert` x) `insert` (x `delete` acc))
                     (Nothing, Just y) -> (c,(l `insert` y) `insert` (y `delete` acc))
                     _                 -> (c,(l `insert` (singleton r)) `insert` acc))
        (let set = fromList (Prelude.map singleton variables) in (size set, set)) rules