9 weeks post op FUE, I still have 15% of the grafts continuing to grow, is this normal?

What you are experiencing is completely normal.  They will eventually shed and then re-grow in the future.

The practice of medicine, hair transplant surgery in this case, does not conform to set rules.  You may be one of the few that have early growth. 

While uncommon, having some hairs stick around rather than immediately shedding isn't a cause for concern. The proportion of hairs that shed later and the duration at which they do doesn't have a reliable effect upon the final results of your hair transplant. But if anything, I would take it as a good sign.

When grafts fall out too early I get worried. I never worry when some hang around longer. If grafts have enough tissue and have good oxygen supply, hydration and are handled delicately during the procedure and are given good post op care - a proportion stick around!

This is common but it is unfortunately not live growing hair that you are looking at, but rather the movement of the hair shafts within the hair grafts as they move up through the skin before shedding. Don't worry, your live growing hair will grow in 5-6 months after the transplant was done

Hair goes through its intrinsic cycle of growth and in your case about 15% were in the growing phase.  Yes it is normal to have some of the hairs not fall off and continue to grow.  However, usually most people start seeing growth in 3-4 months after the procedure and continue to grow in for up to 18 months.