I discovered it quite by accident. When I got the power meter I was mid-plan and I wanted to finish the plan using Virtual Power before switching over. In the interim, I connected the power meter to my Garmin Edge out of curiosity (I was expecting to see a difference in Virtual Power numbers verses a power meter and was keen to see ‘real’ figures). What I noticed from the files was that there was a difference, but that difference varied a lot depending upon how easy or hard I was going, or how fatigued my legs were. So looking for some explanation I went to a local gym to use a Watt Bike to see what that told me. Sure enough there was a big difference, at times as much as 60/40 L/R at endurance/recovery levels, but it disappears by VO2 max levels. I injured my knee on the weaker leg 3 years ago and I had to take a lot of time off the bike. I wonder whether the imbalance started then, or whether it’s always existed. There is also a slight visible difference in the size of my legs - one is even skinnier than the other. 
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