it's hard to say for sure... was it stunted, was it genetics? the more consistent you were helps rules out environmental causes (light etc), but not genetics.
feeling the same way about 2-3 plants in my crop... just because others do well in similar circumstances doesn't guarantee they weren't stunted by our behaviours.
This type of stuff is making me re-think my SOP... i want to be able to plant 1.5x 2x as many as i intend to keep so i can toss out any potential runts or shooting myself in the foot, etc etc.