AsNoriuanswered grow question 3 years ago I am running my first ever Northern Lights too. So from Seedsman - it's smallest plants by structure - I ever grew, my average height is about 1 meter and all 3 my Northern stayed below 50, one went absolute midget.
It's genetics and autos generally tent to be smaller.
At same time I run Zamnesia Northern Lights auto and that plant at least is huge as bush, not tall too.
You did everything right, it's just like your brother's , sisters, cousins all are relatives, but all different.
Good luck !