@silvers - I see this post is over a year old but hopefully you are on email notifications for replies and hence, will see this.
I've had 2 marrow biopsies with the first reporting 10% cellularity and the second reporting 5%. The former was done through Cedars Sinai network and the latter was done at NIH, so very reputable places. While my counts were low they are currently normal and hematologists cannot explain why I have healthy counts with only 5% marrow. Unlike you I do not have polycythemia vera but it sounds like we both have extremely low marrow without explanation.