Since I was already working the the MCT for the Pro-1000 (could likely be same media types as for the Pro-4000 I assume), I checked the settings for Highest Density Fine Art Paper (recommended by Canson for Rag Photographique) and the specific media type for Fine Art Smooth (needs to be updated/installed manually through MCT as I recall, not present from start). They are both set to only MBK and largest ink usage, so no difference there to explain what you notice regarding ink usage (especially PBK usage).
Don't know what media types you have used. Some are set to be a combination of MBK and PBK, for example Heavy Art Paper (aka Heavyweight Fine Art Paper). Could that, or perhaps Ex Hvy Art Paper) have been used, and thereby explain usage of PBK for the Rag?
Regarding the perceived blackness I'm sure there is plenty more to this than actual ink levels used, and even what the spectro tells us. As Mark indicated the FA Smooth seems to simply be more deep black looking, despite what normal spectro measurements and ink levels would suggest. Interesting.