ISO 4548-12 determines the non-retained mass of the total test dust used to determine what mass was actually captured by the oil filter. That's the only way to know how much of the total test dust mass was retained, and how much wasn't retained by the fitter.
If that's the case, you'd expect the BR results to be overestimates of the holding capacities. It would affect the inefficient filters more than the efficient ones.
After many passes through the filter, the uncaptured dust should mostly be <5 micron, which makes up 30% of ISO Fine test dust, so the effect of uncounted particles could be significant, but I don't expect it would have a huge effect. Even the least efficient filters should capture most of the dust.
When the Endurance (25K miles rated) and Extra Guard (10K miles rated) have the same low holding capacity, I'd say it's not pretty solid test data.
You're making the assumption that the mileage ratings are based on holding capacity, and not other reasons, like marketing or filter durability. I don't think that's a safe assumption.
FRAM provided the holding capacity for an Ultra XG7317 to a user
in this thread. It was 6.7 g, which is fairly average for filters of this size. The Ultra XG3614 in the BR test was the same at 6.6 g, and that filter has slightly less media area. I'll request the holding capacity specs for the PH7317 and FE7317 from FRAM to see how they compare.
Some of the OEM Honda or Subaru filters that are the equivalent to the XG7317 will have a higher holding capacity than 6.7 g. Using the Ultra instead of OEM in these applications, even with the recommended 6k or 10k OCI, would increase the risk of filter clogging by at least somewhat. On a 20k mile OCI, the risk of clogging could be a lot higher, and this is probably why FRAM doesn't recommend using the their filters for longer than a standard OCI.
I'd think the same sized Endurance would have a little more holding capacity than the same model Ultra, which gives the Endurance that extra 5K miles use rating.
One thing I've noticed from UOA particle counts is that the Endurance seems to be far more efficient than the Ultra for smaller particle sizes. From what I understand, a filter's dP increase is more affected by the small particles it captures than the larger ones.
This could be why the Endurance had a much lower holding capacity than the Ultra in the BR test. There would be a difference in the amount of uncounted particulate in the oil, which would explain some of the difference, but I wouldn't be surprised if the Endurance actually has a lower holding capacity than the Ultra due to its much higher efficiency for small particles. This could also be why the OG Ultra had such a high holding capacity for a filter with such high efficiency at 20+ micron.
For some context, the particle counts I've seen for the OG Ultra average to ISO 23/19/12 over 4 samples. I've only seen one particle count for the new Ultra, at 23/18/13. The first ISO code at 23 is as high as I've seen for any oil filter on a healthy engine, and the second ISO number is higher than average as well. This is comparing them to a sample size of around 50 particle counts done with various filters.
The particle counts I've seen for the Endurance/Amsoil filters average to ISO 16/15/12 over 12 samples, with the worst being 17/16/14. That's a huge difference in the particle counts for 4+ micron and 6+ micron particles, despite both filters being around the same at 14+ micron.