Could have let'em a couple of days more, but I'm still learning a lot on flushing and I want to try to let the NZH tray go for a longer flushing till leaves starts to turn brown/dried in good number. Will see.
About stretching, yes you're right on the stem. It's the more visible part of the plant doing this.
Try to picture a pure sativa plant, grown in its own hot and humid climate. Think about the buds. It has not a real bud, but just some amount of calixes here and there.
That flower shape give the plant the possibility to be in contact with more air, increasing transpiration. It's a must also for fungi attacks, a good airflow prevent spores germination and grow.
Coming back to shape of the flower, basically like the stem, also those tissue can elongate. It's called cellular elongation and it's the same mechanism we can see in the stems.
So basically temp. is the key and it can switch the plant having a more indica or a more sativa structure, and flower's shape.
That's the main reason I prefer to grow short bushy indicas in the summer if I can, and let the sativas for the winter. However a sativa live better in the summer...grow more, live for more months, that's good for the plant...but obviously it's not good for us, searching dense nuggets.
About the stretching itself for a single strain...well there are many factor contributing to have more or less stretch, but for sure temp is one of those. High temps let's say don't help to keep a plant bushy you know...
ps: I tried once a product named bio-control from a Dutch company. It prevent cellular elongation...and it does indeed. With such product we can modify the game, but let's say it's not my style.