I'd suggest you ask your doctor for more details of the tests next time you see him. It's theoretically possible to pick someone up very early so before any of the 20 cells they looked at on a bone marrow biopsy were positive, but with high enough levels to show clearly on a more sensitive PCR test, but it wouldn't normally happen as if the levels were so low there would be no reason to do the sensitive test.. In your case you had an elevated WBC and pretty much by definition if CML was causing the elevated counts then a high proportion of the cells from a biopsy would be leukaemic.
It's possible that the sample used for the cytology (looking at the cells) was spolied and so they couldn't find appropriate cells to look at, but otherwise it's hard to envisage how they would find the Ph+ chromosome and not find leukaemic cells. Is it possible that he said they found the Ph+ but they didn't find any blast cells? That would make much more sense (and be good news) and would be consistent with your relatively low WBC which suggests the disease was caught early.
The question of can you have the ph chromosome and not CML is a very controversial one and has been the subject of lively debate on here before. Some say Ph+ chromosome = CML and if they've detected it then unless you treat it it's only a matter of time before the disease develops. Others say that you can have the mutation and not the disease either because your body has a natural mechanism to control it or because the mutation has occurred in a cell line too far down the chain to be able to take over the world.