Could it be that you have a grafted variety and it is the understock which you actually end up propagating by root cutting ?

The propagation book I have states this. I have not propagated my boysenberries yet.

I found the answer while looking it up - I guess I missed it before

('Plant Propagation Principles and Practices' --- by Hartmann and Kester - third edition)

Page 191

Periclinical chimera
tissues of one genetic composition over a genetically different 'core'

Rubus has numerous thornless blackberry forms in which the epidermal layer lacks the gene for thorniness. Such plants will usually retain this characteristic if propagated by stem cuttings or by tip layering. In propagating such thornless chimeras by stem cuttings, it is significant that the adventitious roots rise endogenously beneath the mutated tissue, with the result that the root system of such thornless plants is composed entirely of nonmutated cells. If such roots are subsequently used as a source of root cuttings, the resulting plants are thorny. Likewise, seedlings developing from seeds taken from the thornless plant are thorny because the gametes are produced from cells of the tissue which originated from the L-II (nonmutated) layer. Some thornless forms, however, do involve the L-II layer in the mutation, and these transmit the altered characteristics to the seedling offspring.

PRETTY COOL!