Is this a mongoid bug or a feature?

gistfile1.rb

Model.elem_match(:embedded => { :a => 1, :b => 2 })
# => #<Mongoid::Criteria
#  selector: {"embedded"=>{"$elemMatch"=>{:a=>1, :b=>2}}}
#  options:  {}
#  class:    Model
#  embedded: false>

Model.elem_match(:embedded => { :a => 1 }).elem_match(:embedded => { :b => 2 })
# => #<Mongoid::Criteria
#  selector: {"embedded"=>{"$elemMatch"=>{:b=>2}}}
#  options:  {}
#  class:    Model
#  embedded: false>

I would expect that to be expected behavior