confused about measure Gr/M2

I confess that i am amateur ... i need help. I am SO confused about gr/m2 that all the indoor seeds contain. I bought 6choco fondue DNA seeds which says 500-600 gr/m2 indoor . i did it indoor with cocopit soil and sensi dung ( sensi grow and sensi bud) with a lot of care under 5X400W MH and then HPS. They became tall about 150 Cm totally healthy and smell were perfect. I tried my best to flow the book "Bible of buds" .however, when i calculate the wight i found i got 90 gr per plant after dry ... but i expect 500 gr!!!!!
My questions are ;
1- did i miss somthing about gr/m2 ????
2- i used 3 gallons bucket for mother plant ... is that a problem to reach the max yield?

grams per square meter does not pertain to how much you wil yield per plant but how much you will yield in a square meter of growing area aka a 3x3 area

those things went wild also a little bit of training and they wouldn't have been so lanky and probably would have beefed up more