This page is dedicated to 40mm and above artificial grasses and they are not all same by any manner. The lower end and cheaper 40mm ones are generally very light weight with sparse blades and normally made from straight blades/yarn which will tend to flatten easily. The better quality 40mm grasses will generally have a heaver weight per square metre and be made with a structured blade such a C or Y etc. The structured blades are normally more resistant to flattening and spring back into shape more easily. Stitch rate and stitch density (the amount of sowing that holds the tufts) will also play a part in determining the quality of the grass. Low density grass will mean fewer blades and it will be easier to see the backing.
There are many more factors that determine the quality of artificial grass including the backing, yarn quality etc.