The addresses are assigned out of the /30, not exactly what you specify.
This should mean that…
172.16.0.0/30 - Client .2 -> Server .1
172.16.0.4/30 - Client .6 -> Server .5
172.16.0.8/30 - Client .10 -> Server .9
172.16.0.12/30 - Client .14 -> Server .13
172.16.0.16/30 - Client .18 -> Server .17
172.16.0.20/30 - Client .22 -> Server .21
Try putting in .8/30 and see if your results are any different.