The difference in the length of treatment is probably due to a difference in the type or stage of cancer. Breast cancer presents in multiple horrible ways, all of which have recommended treatments.
Oncologists have national and international guidelines that they work to which won't vary except in very unusual circumstances. Public clinics in large hospitals can be a bit of a zoo with the possibility that you may see multiple doctors during your treatment. Private oncologists can be very, very expensive.
I went public this time in a smaller regional clinic where there are only 4 oncologists. I have been very happy with the service, but the oncologist I am seeing is likely to move on to the richer seas of private practice. As they do. The private business I used before that were arrogant pirates.