I found this data for the 90mm RB67 lens on page 42 of the RB67 manual here:
http://www.cameramanuals.org/mamiya_pdf/mamiya_rb67_pro-s-2.pdf
It seems to indicate DOF of 1.59m to infinity at f/32 under the 3 meter focus distance column. Its the closest focus distance listed that includes infinity as the far limit. This isnt labeled as hyperfocal distance but I assume that is whats intended.
If so, this indicates that 1.59m is likely half the hyperfocal distance, making the hyperfocal distance 3.18m.
That would happen if Mamiya had selected the circle of confusion at 0.082mm for its RB67 lenses. Thats about 1/1090 of the 89.25mm diagonal of the 56mm x 69.5mm image rectangle on Mamiya 6 x 7cm format cameras.
COC size is somewhat arbitrary. Years ago Zeiss used 1/1730*diagonal of the 35mm film format. In later years 1/1500 the diagonal of the format is more common on a variety of formats. This is discussed here along with a useful table of COC values for various formats using 1/1500 of the image diagonal.
http://en.wikipedia.org/wiki/Circle_of_confusion
Also, note in part one of the manual on p32 the right-hand photo of the focusing scale for a 127mm lens focused at 1.5m (focused much closer than the 4.4m we discussed for the 90mm lens). The lens extension scale shows a setting of about 12mm or slightly greater.
http://www.cameramanuals.org/mamiya_pdf/mamiya_rb67_pro-s-1.pdf
Thats not precisely the same as the calculated value, but its close.
Δ = pf/(p f) f = 1500mm*127mm/(1500mm 127mm) -127mm = 11.7mm
A possible source of error is the actual focal length. That sometimes varies somewhat from the marked value and can distort the calculated answer.
Other than the factors commented on above, I cannot see why the lens increment noted for the 90mm lens would vary significantly from the calculated value.
Formulas are useful tools for answering the questions we ask, but they depend on accurate, mutually consistent input values. If the inputs are somewhat inaccurate, then so too are the answers calculated from them. Nonetheless, the calculations help us get a reasonable idea of whats going on.