I believe you based on a recent experience during Pettit: Form Sprints in ERG mode - #18 by bbarrera - in that thread you’ll read that during Pettit I changed gears from big to little chainring and that made it feel like I was riding down a hill. So based on the alone, it sure felt like that 15 minute interval at 144W was much lower, definitely below 125W.
Everyone claims the difference is between little and big chainring in Erg mode is the difference between optimizing road feel for climbing vs flat roads… So if that is true, why did a recent 35 minute climb at 93% (224W) and 71rpm average cadence feel the same as doing intervals in Erg mode / big chainring?
Will try and test again this week, because so far I’d have to say I agree with what you are observed.