Bonaire,
I can't say I know the full history behind the liquid valves certified to 25% overpressure but at one time you could get liquid relief valves either certified at 25% or 10% overpressure. Those certified to 25% overpressure could be used in ASME code equipment provided their capacities were adjusted based on the actual overpressure experienced. Most valve manufacturers provided the necessary information to properly size a valve rated at 25% overpressure when used to meet code requirements of 10% overpressure.
You may want to look at API RP-520 Part I, 7th edition, January 2000, Section 2.2.1.2.6 which has also has a discussion about this.
As I understand, liquid relief valves offered by most valve manufacturers today are certified to 10% so that should not be an issue now but you need to confirm that with your valve supplier.