Thanks for the replies. I feel one of the possibilities is on the trainer I can focus on my legs, I even look down at my right leg. But on the road, I have to be vigilant. Another thing is tiredness. My right leg is a lot better when it's not tired (about first 20 minutes) on the road. On the trainer I seldom do 20 minutes (too boring).

One thing for sure is my right leg is weaker. Is there a way to know what's the difference between left and right? I feel my left leg is at least 'one gear stronger' than the right. I plan to do some one leg pedalling on the right in coming winter. Some other options?