Id say Fade pretty much said it. Environmental conditions, specific stress during transportation, genetics...they can all cause a plant to grow at different rates. I wouldnt neccessarily think its a different strain unless you are noticing very different stem and leaf composition and color. When buying clones, unfortunetly you never know the history of the mother or whether those clones even came from the same mother.

The good news is, often those slow to start plants can catch up just fine. Ive had plants that lagged for weeks, then whatever problem was fixed or the plants life cycle finally matched the conditions (there you go Zandor), and they took off and nearly caught up.