hunz,
That's the first time I've heard of any complaints about ARA.
I refined gold and silver professionally for over 40 years and know that gold is rarely what it is stamped. One karat under is not unusual. At least 1/2K under is the norm. If it was made before 1980, the pieces could legally be 1/2K under and it was most always more than that. Some items, especially chains, can have lots of gold solder, which can lower the value. Mexican gold is very often over-marked. The same with a lot of stuff marked 22K or 916 from Asia. Heavy class rings often have lead under the stone. I have seen a lot of chains with marked real karat gold clasps, yet the chain itself is plated or gold filled. In later years, the odds of finding fake plated objects with karat markings has become greater and greater. Also, there is always a natural tendency for owners to over-estimate their gold.
How are you testing it? If you are not filing a deep notch into it and using a drop of nitric to determine if it's plated or gold filled or karat gold, you could easily be missing some fake pieces, whether marked or not. Also, I don't trust electronic testers since I have seen them wrong a few times, either through operator error or tester error. A touchstone always gives the right answer, assuming the piece has already passed the notch test. If you are buying and selling everything based just on markings, that's an easy way to go broke. Also, you'll always think any refiner you send it to is cheating you, even if he isn't.
The only way to really know what you have is to melt it all together and either pin sample the melt or cast it and drill the bar. Pay to have the pin sample or drillings fire assayed.