Higher power indoors vs outdoors

So this is something I rarely see anyone experience, but it is something I experience and I am wondering if anyone else gets this.

When I do a ramp test and do the workouts indoors I have no issues and am confident the value is correct give or take a couple of watts on any given day. All 3x20min sweetspot sessions feel fine, as they should, can do all the supra threshold without issues just about.

But, I ride on the road and do sessions on the road and I can’t put out the same power, I would estimate that I am 10-15 watts down on the road. I have a nice 20min steady climb that I do SS work on and I can barely ride it at my FTP taken from ramp test, it feels closer to a flat out effort than a threshold effort.

Couple of notes, same power meter used for inside and outside, calibrated before every ride, different bike, alu inside and carbon outside, identical measurements and contact points. Trainer is an elite drivo dumb trainer.