1st Mammoth kissing the sky.. but why did the rest grow 1/2 the size ?

Out of a package of Mammoth seeds I have 2 sunflowers around 13' feet tall , the first one just opened and the other looks like it's a week behind.. But the question is why did the rest of seeds grow to around 5-6' , bloom , and are almost ready to be removed as compared to these 2 ? Did I plant them too close? Or was it the seeds ? ( First time so a bit confusing.. TIA