I think it is because we imagine the cancer growing inside the prostate and then bursting out of the covering to then climb across to a lymph node or a bone. It doesn't work like that.
The cancer can burst out of the gland and invade other things close by, such as the bladder, bowel or pelvic muscles. Or it can stay in the prostate and look quite small but blood and lymphatic fluid both travel through the prostate and it is possible for the flowing liquid to pick up cancer cells and then carry them away. If the lymphatic fluid picks up some cancer cells, these will usually be collected in the nearest lymph nodes (which act like a sieve, I suppose) - this may be what has happened in your case. Or the nearest lymph nodes can't collect it all and the cancer cells are then travelling all around the body - this is N1 in a diagnostic report and the genie is out of the bottle, the whole lymphatic system cannot be treated curatively. Or the lymph nodes do their job well but the blood carries cancer cells around the body, including to the bone marrow where cells settle and then metastasise. Again, the genie is out of the bottle and generally speaking, although one or two bone mets can be zapped, there is a high chance that the cancer cells are already settled in bone marrow elsewhere around the body.
What is interesting is that for most men with PCa, the prostate cancer cells travel around the blood or lymphatic system but are never able to take hold / metastasise. I think PCUK was funding some research at one point to try to identify why some bodies can clean up the travelling cells and some can't.
I don't suppose there is any way for you or the surgeon to know whether those cells had all been caught in the sieve of the nearest lymph node (in which case, he should have a lovely undetectable PSA for the rest of his life) or whether the cancer was already settling in other nodes around his body. It is a waiting game and, if you are lucky, it will be a very long wait - my dad had his recurrence 13 years post-op although obviously those tiny micromets had been sitting there all that time.