David - the problem with many such sites is that the writers do not understand the most basic principles of physics or electronics, have a particular blind spot regarding how test equipment actually works, and thus have no idea when they stray from the possible to the comical. For example, on one of the pages the writer connects an HP frequency counter's reference output to its input to measure the reference, and is amazed that it measures perfectly. It's the reference. It always will measure perfectly, even if it is broken.
At it's core, all test equipment performs some form of a comparison between an internal reference and an external signal. Voltmeter, frequency counter, bathroom scale, whatever, it does not matter. ALL measurement depends on a reference value or quantity of some manner, and the quality of the reference is the single most important factor in the quality of the instrument. Therefore, when the instrument measures its own reference, the result always is perfect unless the instrument is seriously busted.
For example, if the reference oscillator actually were 9 MHz instead of 10 MHz, it still would measure itself as 10 because it thinks it is a 10, and it would measure an external, true 10 MHz source as 11.111111 MHz. This is 5th grade arithmetic; if you have not mastered this, your results will vary.
If you really want to know the accuracy of your frequency measurement system, purchase a rubidium frequency standard (a low-cost atomic clock), turn it on and let it settle for a few weeks, then measure its output. SRS has a nice one for $1500 plus the cost of a decent power supply.
ak