I think splash breed true- when bred together they produce randomly patterned white and blue spotted offspring.
Its that the pattern and tones can be inconsistent, therefore they are thought to be hard to standardize.
I wouldn't want to let a technicality discourage anybody from raising splash though. Its so trivial.
When splash are bred together they lose their spots becoming basically white or very nearly white. As Walt explained earlier, however, it is expected that blue birds are regularly injected back into splash breeding programs in order to keep the coloured "splashes". (Being a blue girl, I LOVE the splash. I deliberately work to get it because it's like a crap shoot. Just never know what might pop out of that egg!!

Lavenders are also tricky because lavender to lavender very quickly leads to feather problems. So black has to be used in a quality lavender breeding program as well.
Same thing with Frizzles - but now I'm REALLY wandering ....
