The term is commonly called "het" short for heterozygous. It can't be 25%/75% because of the basics of genetics, you have two choromosomes, and you cannot split those into 4.

In some cases, traits are recessive, which means that if a BP is 50/50, it will be a carrier, but will not display the traits. Those snakes are probably the closest to the ones you're talking about. For a recessive, you would need to have both chromosomes with the trait in order for the trait to display.

I'm not familiar enough with all the BP dominance/recessive traits, but I believe the Spider trait is recessive. Therefore, if you mated a normal and a spider, you would get a snake that was 50% Spider, 50% normal, but it would appear to be normal.

If you mated one of these snakes with a pure spider, the resulting snake would not be 75% spider, rather it would either be 100% spider or 50%spider/50% normal because you get one chromosome from each parent. The spider only has two spider genes, so it definitely gives a spider, the normal parent either gives it's spider chromosome or it's normal one. That's how you get the results.

So hopefully that explains why it can't be 25% and 75%. 50/50 is possible though.

If any breeders want to correct me, feel free, I'm attempting to explain from a pure biology stand point without the specifics of BP traits.