I have a different theory to yours regarding resolutions/pixel density.
The new 6.1” OLED (6.06”) takes the current 6.5” (6.46”) resolution:
6.06” @ 2688x1242 = ~489ppi (163dpi@3x, note that 326ppi is 163dpi@2x)
The new 5.4” OLED takes the current 5.8” (5.85”) resolution:
5.48” @ 2436x1125 = ~489ppi (same scaling as before)
The new 6.7” (6.67”) OLED takes a new resolution:
6.67” @ 489ppi = ~2964x1368
This brings them all to the same pixel density and back to the original scaling of 163dpi which the XR uses, so that no screen real estate is lost on the smaller 5.4” phone or on the new 6.1” compared to the XR. The new 6.7” merely falls in line for operational consistency and to maximise the benefit of the larger display.