Lindsay, I'm a relative orchid newbie, and the whole name thing baffles me, I must say. But my interpretation of your question is, why is one Blc. tall with long leaves and another Blc. is shorter with rounded leaves, right? Maybe I can try for a more basic explanation.
So, there are thousands of different Brassias, Laelias, and Cattleyas, as I'm sure you know. Some are big, some are small and they flower in lots of colors. So if one cross had parents that were small plants, you'd expect to get a small plant. The other cross may have had larger parent plants with longer style leaves, thus you got the taller traits.
Going with your dog analogy, if you had a really BIG Lab crossed with a standard poodle, you'd expect to get a really big Labradoodle. If you had a small Lab crossed with a mid-size poodle (if that is possible) you'd expect a Labradoodle that might only be knee-high. But you might get a bigger dog if one of the grandparents was big. Even if you had two puppies from the same parents, they aren't always the same size, shape or temperament unless they're identical twins.
It's the same with orchids. I've read that when they breed an orchid, they can't get the name registered until they've crossed it enough times that the traits (size, leaf shape, flower color and form) come true with every plant. Another reason that named, registered orchids are so expensive - there's a lot that goes into the breeding, and years of growth to get it to breed true, then to a size that you can buy it in bloom.
