I have noticed this as well recently - RT being cheaper than 2 OW. I've noticed it with direct flights only, not with layovers. YMMV obviously depending on route(s) and date(s).
Couple recent examples...
PHX-LAX-PHX and PHX-SFO-PHX cheaper booking RT than 2 OWs both in May. PHX-MIA-BWI-MIA-PHX same price - doesn't matter if RT or 2 OWs - this is for later in the year August/September.