There are many reasons one is shorter than another.
Some are taller than others.
Some may be shorter because they get more sun. They can be in the same area and another sedum in front of it can cause it to get a little less sun. If it get less sun they tend to be taller because they stretch for the sun.
I have some in my shade garden because I like the fall color. Those are as tall as a bush in shade.
Yes they do start getting their bloom heads early especially if it is very warm. So it make it seem like the blooming heads are there a long time.
