The times are designed to give a specific distance between a/c; ie 10nm, 15nm etc. Ideally everyone would get it perfect, and it would all work. However, pilot preceding may be +30 seconds, so pilot following, although spot on time, will need to slow down/ be vectored to meet the required spacing. Add to the mix differing a/c types, speeds and altitudes and there are just too many variables for it to work perfectly every time.