MongoDB Index bound behavior

db.ensureIndex( { 'a.b':1, 'a.c':1 } );
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Mongo > db.version()
2.4.1
Mongo > db.test.find({ a:{ $elemMatch:{ b:1, c:20 } } } ).explain()     // index bound works perfectly
{
    "cursor" : "BtreeCursor a.b_1_a.c_1",
    "isMultiKey" : true,
    "n" : 0,
    "nscannedObjects" : 0,
    "nscanned" : 0,
    "nscannedObjectsAllPlans" : 0,
    "nscannedAllPlans" : 0,
    "scanAndOrder" : false,
    "indexOnly" : false,
    "nYields" : 0,
    "nChunkSkips" : 0,
    "millis" : 0,
    "indexBounds" : {
        "a.b" : [
            [
                1,
                1
            ]
        ],
        "a.c" : [
            [
                20,
                20
            ]
        ]
    },
    "server" : "bdvlpabhishekk:27016"
}
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Mongo > db.version()
2.2.0
Mongo > db.test2.find({ a:{ $elemMatch:{ b:1, c:20 } } } ).explain()    // check the index bound, not working as expected
{
    "cursor" : "BtreeCursor a.b_1_a.c_1",
    "isMultiKey" : true,
    "n" : 0,
    "nscannedObjects" : 1,
    "nscanned" : 1,
    "nscannedObjectsAllPlans" : 1,
    "nscannedAllPlans" : 1,
    "scanAndOrder" : false,
    "indexOnly" : false,
    "nYields" : 0,
    "nChunkSkips" : 0,
    "millis" : 0,
    "indexBounds" : {
        "a.b" : [
            [
                1,
                1
            ]
        ],
        "a.c" : [
            [
                {
                    "$minElement" : 1
                },
                {
                    "$maxElement" : 1
                }
            ]
        ]
    },
    "server" : "bdvlpabhishekk:27017"
}

---------------------
Mongo > db.version()
2.4.1
Mongo > db.test.find( { 'a.b':1, 'a.c':2 } ).explain()      // check the index bounds, not working as expected
{
    "cursor" : "BtreeCursor a.b_1_a.c_1",
    "isMultiKey" : true,
    "n" : 1,
    "nscannedObjects" : 1,
    "nscanned" : 1,
    "nscannedObjectsAllPlans" : 1,
    "nscannedAllPlans" : 1,
    "scanAndOrder" : false,
    "indexOnly" : false,
    "nYields" : 0,
    "nChunkSkips" : 0,
    "millis" : 0,
    "indexBounds" : {
        "a.b" : [
            [
                1,
                1
            ]
        ],
        "a.c" : [
            [
                {
                    "$minElement" : 1
                },
                {
                    "$maxElement" : 1
                }
            ]
        ]
    },
    "server" : "bdvlpabhishekk:27016"
}
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