If you examine carefully the way the pins and jacks mate, you can see that there is a fair amount of play in each mated pair. The ATX plug from PSU#1 may fit fine in mobo#1; but a couple of pins in the plug from PSU#2 are slightly off. MBM5 results would favor PSU#1.If
the tester's motherboard pins have deteriorated from wear then I agree they certainly shouldn't be testing voltage via the motherboard under any circumstances, since any faulty connection can't be attributed to a potentially poor PSU connector - there is no way of knowing where the source of the error is. However as an end user it's useful to me to know that what comes through the plug when it's plugged into a "proper" motherboard connector
is at the right voltage level; testing of only
the output at the tips of the plug can't tell you that - you would need to run you run the voltage test through a quality mb connector part first or the quality of the PSU connector would remain unverified. A great power supply with lousy connectors isn't much of a bargain....
Then there is the question of when the tests are conducted. If they are done at the same time, then it's a bit better; but if you have a gap of more than a couple of weeks, I guaranteed you a test review system will have been assembled & disassembled enough times to have at least a bit more contact deterioration.
I thought that likely, the ABBA reference was meant to informally convey a counterbalanced research design (measure A, then B; then B, then A; average both) which controls for time-dependent effects...
Another thing: a single screen shot of the MBM5 tells you almost nothing at all. You really need to see what the voltages are doing over a period of time (30-60 mins) under a variety of loads, with sampling being done as often as possible
Mikhail's site, among others, isn't doing a snapshot approach - from their Seasonic tests: "I put each power supply through two different testing sessions. The first session was a one hour of the Prime95 torture test followed by an extended period of idling in order to measure the voltages from the power supplies". The screen shots of MBM shows the high, low, and average
voltages derived from hundreds of samplings taken over the test periods....probably the major reason (aside from ignorance) that MBM is used, since as you say doing this by hand would drive you nuts!