very high , high,normal,etc are relative to each other............
they will make difference when say there are 3 tasks running - one with very high , 2nd with high and third with normal.....then the difference will be evident....
if you have only one task...then it doesn't matter whether it is very high, high or normal.........
or
if there are three tasks all with high priority...even then priority wont show effect coz relatively they are at same priority level which same as having all the three tasks at normal priority