performance - Mongodb query using $and operator does full scan while same query without doesn't -

i'm relatively new mongodb , i'm having trouble understanding why query $and operator seems full scan, while same query without doesn't.

my document looks this

{_id:"123456", labels : [{label:"beef", language:"en"}, {...}],...} 

i have compound index on label , language, however, explain shows different results depending on whether do

db.mycollection.find({ "labels" : { "$elemmatch" : { "label" : "beef" , "$and" : [ { "lang" : "en"}]}}}).explain() {     "cursor" : "complex plan",     "n" : 4,     "nscannedobjects" : 0,     "nscanned" : 16701573,     "nscannedobjectsallplans" : 0,     "nscannedallplans" : 16701573,     "nyields" : 130540,     "nchunkskips" : 0,     "millis" : 16283,     "filterset" : false } 

or

db.mycollection.find({ "labels" : {$elemmatch: { label:"beef" ,"lang" : "en" }}}).explain() {     "cursor" : "btreecursor labels.label_1_labels.lang_1",     "ismultikey" : true,     "n" : 4,     "nscannedobjects" : 4,     "nscanned" : 4,     "nscannedobjectsallplans" : 4,     "nscannedallplans" : 4,     "scanandorder" : false,     "indexonly" : false,     "nyields" : 0,     "nchunkskips" : 0,     "millis" : 0,     "indexbounds" : {         "labels.label" : [             [                 "beef",                 "beef"             ]         ],         "labels.lang" : [             [                 "en",                 "en"             ]         ]     },     "filterset" : false } 

can me understand why?.

thanks in advance!.