Mass/energy ratio is different. There is not enough energy in RAS sacu to convert all available mass into Para.
Para cost 250k mass and 7500k energy. Ras sacu 6.5k/117k * 0.9 = 5.85/105 (sacrifice multiplier 0.9 for sacu, 0.6 for engies)
In terms of mass you can get: 5.85/250 * 100% = 2.34% of para
Energy: 105/7500 * 100% = 1.4%
So in the end 1 sacu gives 1.4% and all unused mass dissapears. Pretty weird and unintuitive system I should say, but at least it's 100% bug/abuse-free