Accidental Cross vs Sport vs Instability

[bower]

If I found one RL in a batch of PL's I would think a crossed seed is the more likely. Since an F1 cross with an RL parent would come up RL.
If you grow it out and fruit is identical to Sweet Sue then, maybe a sport.
Grow it and see, is the only way to figure out what happened.
Also with New Big Dwarf, not-dwarf is dominant to dwarf. So in an accidental cross, the F1 generation will not be dwarfs, if one parent is not a dwarf.

Re: 25%, you're thinking of F2 generation and recessive traits. In the F1 - first generation of the cross - the dominant traits will show in all of the crossed seedlings, that is any dominant trait in one of the parents. - RL, indeterminate, red, etc etc.