Good memory! Here's the breakdown based on fares from the Dec 201
8 fare chart: 3 low bucket Coach fares were 3 X $110 = $330 plus 2 low bucket Roomettes were 2 X $234 = $468 for a total of $330 + $468 = $798.
So far, so good.
While your $910 total is correct, the fares totaling it are off because the $90 Coach fare you used is the Saver fare and (provided there's no sale in progress) the Coach fare that goes with any sleeper is two buckets above the Saver fare. For the LSL or CARD that's currently $144 for each person (Coach fare with the > next to it on the chart in Post #52). Not sure what your ". . .shows $320 being the calculated room cost. . ." refers to but the current low bucket Roomette upcharge is $383 - $144 = $239 based on the data in Post #52 (in which the Coach fare with the > is included in the fares for all sleepers).
So all this makes the current breakdown for three adults in two low bucket Roomettes: 3 higher bucket Coach fares is 3 X $144 = $432 plus 2 low bucket Roomettes is 2 X ($383 - $144) = 2 X $239 = $478 for a total of $432 + $478 = (ta da) $910!
The increases were thus 31% in Coach fares ($110 to $144), 2% in Roomette upcharges ($234 to $239) and 14% in total fare ($798 to $910).
FYI, the first several years of fare bucket charts showed just the upcharges for sleepers (no Coach fares included) because the original version of AmSnag reported them that way. When version 2.2 of AmSnag came out it showed 1 (or 2) adult Coach fares with any sleeper and I stubbornly deducted the low bucket Coach fare from the total sleeper fares so I could keep on as before. But those subtractions (as many as 185) got to be tiresome and a possible source of error . So starting with the 12 Nov 2019 version of the chart all sleeper buckets include one adult Coach fare, the one with the >.