here is the easiest way to look at it..

when breeding a 100% het x 100% het you get
25% normals
50% het's
25% homo

Since you can't tell the normals for the hets 75% of the clutch is normal looking.
So of the you take 100% of the normal looking animals and since they were statically 75% of the total clutch you divide 100/3 this gives you 33.33%. So say you get 3 normals each normal on its own has a 33.33 % chance at being normal this means that of the 75% of original 100% (or 4 possible outcomes) 2 of them will be 100% het's.
So out of the 3 normal looking outcomes or 100% of the normal looking animals you subtract the 33.33% chance of it being just a normal and your left with 66.66% chance each animal will be a het.