You are right, yes there is a science behind this. Actually Shell Advance AX3 is 0.75L oil but on the bottle it's ,menitoned that it is 0.7L. For it's proof if you see the bottle of YAMALUBE which is same oil and the size & design of bottle is also same shows 0.75L while the Shell's own bottle shows 0.7L.
Actually the companies count the oil in form of digits 0.2L, 0.4L, 0.6L, 0.7L, 0.8L but not 0.75L so the same in case of ZIC.