A "typical" relief valve has pressure override. IOW, if the amount of oil dumping over the relief is more than expected then the pressure is higher than expected, potentially quite a lot higher depending on the characteristics of the system.
A cause for that could be as Woody eluded to. Restricted oil flow somewhere could/would cause higher flow over relief causing higher pressure override.
I deal with a fair amount of hydraulics and typically the relief pressure is at cracking pressure. Cracking pressure is with a small amount of oil flowing through the relief valve. Something like 1/2 to 1 GPM isn't unusual. However, if there is say 5 GPM going over relief, the pressure could/would be considerably higher.
Last edited by whoaru99; 11-15-2012 at 10:05 PM.