This page: http://docs.mongodb.org/master/core/multikey-index-bounds/

It should mention the following limitation. If there is a compound multi-key index including a non-array field, then if this field is queried (and even if it's a leading field in the index and if queried alone with no array field) then MongoDB will not be able to intersect the bounds properly. And $elemMatch will not help because it is designed to work with array fields only.

The only workaround is to create a non-multi key index on the non-array field and use that for the range queries.

The situation is explained in detail in SERVER-6720.