Already answered your personal mail .
Main thing was that J behind the loadindexes stands for speedcode J = maximum speed of tire 100km/65m/h. They sayd loadrange J but 16PR on sidewall stands for H load.
Also the small difference between single and dual is strange, rule is maximum load dual = 92.5% of Single maximum load. If you remove the rear loadindex from the spreadsheet ( red arrow front but then rear) , it is calculated from front with this 92.5%. Then the advice behind will be higher because of the lower maximum load dual.
Fact also is that the list of Doublecoin gives lower maximum load for the sise.
This can be explained that that list is for higher speedcode of for instance M= max speed of tire 130km/80m/h.
Filled in the spreadsheet of mine, with your optimistic given tire data. and here a picture.
Mind the maximum speed then of 65m/h and higher pressure would give more reserve for the speed. Calculated back the axle loads given by typing "=4*3375 "
then then it is calculated in the spreadsheet.
These loads look a bit like you used the GAWR. Is that true or are they weighed loads? If so replace the loads to part 1 GAWR's and leave given load empty.
You can also download by RIGHTCLICKING the spreadsheet of my examples map
Then you can chanche it all.
conclusion at the moment, Doublecoin did some creative calculating , to give the tire a better place in the market, and probably did some things wrong in that.
Have to look back in their list what loadindex they use there.