MongoDB takes different time for with and without

Question

I know, I know, hint forces MongoDB to use a specific index.

I have two queries:

db.mycol.find({sourceId:ObjectId("596bac5a6f473e1a042bFFFF"),myFlag:false}).count()

db.mycol.find({sourceId:ObjectId("596bac5a6f473e1a042bFFFF"),myFlag:false}).hint("sourceId_-1_myFlag_-1").count()

Both of these commands take different time:

Without hint: 4-6 seconds
With hint : 0-1 second

However, both of the commands are using the same index:
{ "sourceId" : -1, "myFlag" : -1 }

When I execute db.mycol.aggregate( { $indexStats: { } } ), I can see the accesses.ops count increases for the same index.

Even explain shows me the same index will be used!