The only reason the 2 combos look different on WOBP is probably due to the parent morphs looking different and/or on different ends of the spectrum. For example, the lesser can have a reduced pattern, while the butter have a busy pattern. And the fire in the nuclear vs fire lesser could look different and contribute differently in the offspring. Like one fire can be really blushed and flamed out, while the other is a sub par fire w/ little flames and blushing.
If you got two similar looking butter/lessers and bred them to the same fire, I bet the offspring will look alike.
And offspring in a single clutch can vary greatly as well..
In my opinion, a quality lesser can produce the same stuff as a quality butter.