We have done lots of tests like that using samples from our own horses. Worm counting is not an exact science as I'm sure you appreciate, but it is 'good enough' and certainly the best tool we have to help us reduce dosing and get the treatment correct for those who need it.
Testing several samples from the same dung heap has produced similar results for us. The numbers won't be quite the same but you could definitely see low, middle or high levels of worm eggs. So you might get 350, 500, 300, 650 but these are all medium counts which would require worming. What is more important is the method used for sampling and then for testing.
So samples should be taken with tiny pinches from several places in the dung pile and pressed together in the sample container, filling it to the top to exclude air.
The test should involve spinning the samples in a centrifuge in the lab to give a more accurate result. Simple strained methods will not capture as many eggs.
We once tested samples from a yard and found numerous medium counts. Due to a mix up the yard was also tested by a local person who goes around visiting yards though I don't know which method she was using. She pronounced them all clear!
All our tests and results are reported honestly so you would not need to send under different names if you wanted to check us out. Our mission is and always has been to improve knowledge and practice of horse worm control among owners.
Just to say, the new tapeworm test is a wonderful invention. We are doing a study on our horses which is proving really interesting. Most of them were fine but my pony had an unexpected high tapeworm burden. As he had been treated with praziquantel in March it was quite a surprise to find this. At least I could deal with it once we knew it was there. I would recommend the test to everyone.
I hope that helps. Keep routine testing instead of routine worming.