I’ve noticed this particularly with Lurkers.
Lurkers will be listed as Hard+ in the enemy information pane. Yet, we’ll often get Lurkers (and sometimes some other Hard+ units) if both of us are queued for Normal.
Hard is the threshold where you start getting cloaked/burrowed units, as well as see spellcasters and splash damage start to be used. Sometimes a unit will be available in Normal, but won’t behave like a Hard+ unit (for example, unburrowed Swarm Hosts, uncloaked Ghosts that just a-move, or High Templar that just shoot water balloons).