I’m just trying to work this out myself. My hm numbers are way higher than my ff. Just did a ff and my map and ftp dropped >20%. I’ve been trying to work out what why etc etc. So work has been super stressful, a race I’ve been training hard for was cancelled because of rain and maybe I just screwed up my taper. Who knows. But if I ride 9 hammers on my ff numbers - easy peasy. If I ride on my hm numbers - I fail on #9. The hm numbers “feel right”, and interestingly end up very close to what I manually adjust erg numbers to, to achieve the rpe.
But… My “thing” is long hard efforts. I like riding 6-7 hours just below ftp. So I think it might be more complicated than what mac says. I’m definitely not a sprinter - my mate is… my profile usually shows TT or rolleuer, unless I blow up like this time. When I blow up it says sprinter which I know I’m not. I’m much faster over 200km than I am over 500m.
I think you just need to keep playing and testing and adjusting and working out what actually works for you.
And then you’ll have days / weeks etc where stuff just doesn’t work, and maybe your results are just whacky. (I suspect my last ff false into this category)