difference in kelvin temps,,, ive found this to be true also... i used to veg w 6500k mh then bloom w hps 3000k lots of stretch,, but i think this is the norm,,seems to me most growers use this mh veg hps flower and the stretch to an advantage.. then veg w 6500k mh and bloom under a mix of 3000k hps and 6500k mh, less stretch and better potency... i now have been using the 3100k cdl light for both veg and flower,,in veg i get shorter plants w/ more nodes, and in bloom plants rarely double their height,, i wanted more stretch,, i added to my bloom 2 100 watt citizen cob 4000k to my 2 3100k cdl's,, now i get the stretch i want... i know temp's play a role in this,,however my room is temperture controlled lights on temps near the floor 72*f temps at plant canopy or up by the ceiling 75*f never above,, lights off i keep the room at 62*f at the floor but the plants are at 65-68*f,, i have intake's exaughst fans and multiple oscilating fans pluss one blower that sits on the floor continually blowing the air up,,,even w all these fans there is still a temp difference from floor to ceiling...