Check this out.
https://cyclingtips.com/2013/09/clim...-are-affected/
I have always been intrigued by the differences of perceived effort between climbing, flat riding, time trialing and the trainer. According to the article it is about the way muscles are recruited on the different types of terrain and the trainer.
On the track and time trialing at higher speeds one is constantly topping up the energy to maintain speed where as when climbing or on the trainer, one recruits muscles differently to overcome a more constant force such as gravity.
There is no bad news here. My interpretation is that practice makes perfect and different muscle compositions will favor different situations. The only wild card is interpreting test data i.e. comparing riding the trainer versus riding on the track or the road.