Those ink limits are significantly lower than the optimal ink limits on the 200 series printers with 792 series ink. The ink limits are also lower than HP media profile limits for those materials for the 200 series. Maybe the optimizer causes problems at normal ink limits so they had to reduce how much ink is laid down. Perhaps the ink + optimizer limit is close to the media limit. Or possibly some person at HP just set the limits arbitrarily low.
Having made countless profiles on all grades of adhesive vinyl, I have found the HP vinyl to hold the least amount of ink while premium vinyls will easily hold over 50% more and have much better color. If HP is using their mediia as the baseline for the hard-coded ink limits, then the low limits in that chart start to make sense.
if you turn off the optimizer, do the ink limits go up?