OK so you've verified pressures are accurate by swapping out gauges (and still using the same adapters?). That's one good thing, we know it's accurate.
Pressure at the sandwich adapter is going to be different than pressure at the OEM switch location (also a common gauge location), and different at the top of the block. Descending in pressure respectively.
The top of the block is after the second split off of the primary engine feed passage. So, imagine the feed goes from left head across the front, where it feeds the block, and then to the right head. That top passage is a bit away from that main feed. Essentially you are tapped into the line that feeds the mains.
The pressure drop is more than I would expect, but we've still yet to perform our BRZ oiling testing
It could be one or more of several thing. There needs to be more info to determine if it's a blockage. Above 5,000 RPMs? What is the differential at other RPMs? A chart at 1,000 RPM intervals would spell this out. Could be a bleed in a seal, or a bearing issue. The latter being a log shot IMO.
Has the setup always done this on all engines? If so, probably normal? Or, is this a more recent phenomenon? How does the car run? Any issues with oil consumption or oil temps?