What ever is in the feeder is send according a semi-equal weight rule, presently. Since HCMD2 on average runs 4.6 hours and NRW run fixed 7 hours and HFCC current 7.8 hours etc, it's a juggling act to balance the quantities so that multi-project devices get approximately same run time over a longer period. Since HCMD2 is rather variable as batches work through parents/children/grandchildren, it is conceivable that the numbers of WU currently translate to more hours than the project average, but if you look in the below chart, projects on the whole are not far apart in total run time:

Full Size Image Link

Those that have a preferred project specified get first serve, that might be RICE in your case, so the mix in the multi project pool can vary in favor of one or the other project in individual cases.

edit: someone/somemany, those in the 2nd Annual FightAIDS@Home Challenge, must have taken all this projects WU's as on the HCMD2/RICE/FAAH mix profile I ain't getting any ;>)