I think you could assume that it could be both. Since you are using two lights, then your spread will defiantly be different in the middle vs right under the light. So yes, the light could very well be the culprits.
But, you cannot throw out the chance that it is a strain as well if you are running different strains. They can totally behave different. If these are all clones then you can safely say it would be the light.
I hope this helps.