That's because in their testing, they use an isolation transformer and float the power supply! In other words, they work hard to remove mains leakage from the equation.
That is incorrect. Plugging an SMPS into an isolation transformer does absolutely nothing to eliminate the transfer of AC leakage to the DC -ve (zero-volt "ground") of its output.
John Swenson ran a whole series of direct leakage current measurements (using a differential probe and a proper impedance loading circuit) on a dozen different SMPS units. All of his tests were with the units plugged into a 2.5kVA Topaz Ultraisolation transformer (with 0.005pF interwinding capacitance).
Here are direct measurements of the iFi iPower 9V/2.0A unit at three different bandwidths (1kHz, 50kHz, and 1MHz):
For comparison, here is the AC leakage from a Mean Well GST40A07 (7.5V/5.34A) run with the same setup (1kHz bandwidth):
As we have explained numerous times elsewhere, the pernicious high-source-impedance form of leakage (what all the above are measuring, and which yes, Amir's early test results with our original UltraCap LPS-1 into single-ended DACs prompted us to research this lesser-known form) is easy to get rid of--simply by shunting the DC -ve ("ground") of the SMPS output to AC mains ground. So here is the same Mean Well SMPS with that done:
And with the same test set and the same "ground-shunted" Mean Well used as the AC>DC charging supply, here is the 1kHz leakage from the output of our original UltraCap LPS-1:
By the way, the new generation UltraCap LPS-1.2, with uniquely paralleled LT3045 regs and wider 5/7/9/12V output selections, comes with an UpTone-branded 36W SMPS charger that is already internally "ground--shunted," so high-source-impedance leakage never enters our piece. Mean Well units are not designed that way and they failed to understand/comply our request, even for 1,000 units, so I found another reputable manufacturer whose 3-wire units are already designed that way.
(SMPS manufacturers only understand the traditional low-impedance "touch current" leakage--which has ALWAYS been 100% blocked by our UltraCap units, but the few tens of picofarads of total capacitance of the transistors we use for bank-alternating--instead of mechanically noisy large relays--is the path for the high-source-impedance form to pass through. As shown above, prevention of it ever entering is an easy thing to do.) And we offer our new internally ground-shunted chargers for $15 to original LPS-1 owners. Not that anyone is able to hear that difference, but it is technically better.