Yup my experience matches Anne's, some seem in a hurry to be Monster, others take ther time. It's mainly genetic, but without the right environment and diet, that will affect them a lot too.
Endlis growth rate can varied, depend on the food, environment and the parents gene. Some grow to 14"-15 in the first year, some 18"-20"+ in the first year. Average would be 16"-18" in the first year for regular length endli and semi-short body around 12"-14".