Those are indeed the volumes
used to make a cubic foot of the mixture.
If you add them up, you'll have a total volume of 1.54 cu. ft. but you are looking for a 1 cu ft.
So where is the difference?
It's because when you mix the parts, the smaller granules fill the place taken by the air, which is expelled, diminishing the total volume.
Then you take your previous values to exclude the air:
y= .44*.66 =.290
z= .88*.55 = .484
Then x+y+z=1 cu. ft.