I'm looking at making a LHR-PHX booking on Feb 14. I'm not sure about my return date yet so was thinking of booking one-way. Then I noticed a stark price difference for the LHX-PHX depending on whether there's a return and the date of said return (lowest economy fares in USD):

LHR-PHX on Feb 14 (one-way): $2,500
LHR-PHX on Feb 14 (returning Feb 19): $990
LHR-PHX on Feb 14 (returning Feb 20): $620

I thought AC had decoupled fares a few years ago such that booking one-way vs. return made no difference. Am I imagining that?