Splicing refers to the processing of mRNA, in the nucleus, after The AR Transcription Factor binds to the template strand and the RNA-polymerase moves down the template constructing the "precursor" mRNA.
Since the AR variant 7 is referred to as a splice variant, this means it is not related to a mutation of the AR gene itself, but to its processing (editing).
Does this provide any clue?
It almost seems as if section 7 (the DNA Binding Domain) is specifically deleted, but the later sections farther along the template strand are retained. This seems to be not at all random, but I assume a normal part of an evolutionarily developed survival technique, or reaction to local conditions in the nucleus. (It has to be conditions in the nucleus, because that is where this is taking place).