Where are you specifying the probabilities?
If you don't already have it, download this NESS guide. From that, there's this (~page 56):
"If the general formula for the flag is LtnAmBqCr, then:
· the probability of three items in inventory of creature/container = r% (parameter of C),
· the probability of two items in inventory of creature/container = r% + q% (parameter of B+C),
· the probability of one item in inventory of creature/container = r%+q%+m% (parameter of A+B+C), and
· all items will be randomly selected from merchant named LOOT_n.
LT501A30B20C10 would put three items in inventory 10% of the time, two items 30% of the time (20%+10%) and one 60% (30%+20%+10%), and none 40% of the time from merchant LOOT_501.
LT502A0B0C100 would put three items into the creature/container inventory 100% of the time. The parameters on the A and B subflags are irrelevant in this case.
LT503A100B50C50 would put three items on the creature/container 50% of the time and two 100% of the time; the parameter on the A subflag is irrelevant.
If no A, B, or C subflag is used, the defaults are A50B15C5. "