It is a little confusing because of the history of how referrals evolved.
Initially, there was only the Alpha referral rewards - as you point out - 9 NRVE per referral. For many people, this is not much of an economic incentive and I think many of the people making referrals are doing so simply because they believe in the platform rather than for monetary gain.
Then recently, the Team decided to dedicate some of NRVE that will be unlocked after Beta Launch, to what I can only assume will be an expanded referral program. Perhaps with higher NRVE incentives. The question that is probably on their minds is: how much is a new member worth to us in the critical growth stage of the first month? The second month? Months 3 to 6? Second year of operations?
The top 10 referrers may not be growing their referrals anymore simply because they have done all the outreach they can think of doing, and are coming up against the law of diminishing returns.
I only referred 9 people in the first few months of the referral program. It is only recently that I've started putting a bit more effort into trying to help Google + communities discover Narrative. Check out the G+ Narrative community here:
I don't care about the NRVE rewards, but I love this project and would like to see it grow.