Not worth worrying about, especially if it has a few miles.
In an IDEAL world they would all be within 10% of each other, but I wouldn't personally bother about 18%, as it means stripping bits down.
You could always do a dry then wet test to see if it is rings of valves, it is most likely valves,(or just the tester did not make a good seal on the low one).
What car is it anyway.
tabetha