Your earnings for referrals will vary depending on the following factors .
Impressions:

When a referrals through your referral gets 100 impressions on his urbanread news page your Adsense* ad gets 20 impressions.
If the publishers gets higher number of impressions your Adesense* ad impressions will also increase.
The exact number of impressions is 20 % of what your referral gets.
The higher number of impressions will result in more possibilities of earnings.
No of referrals:

When the number of referrals you have made increases obviously the number of instances your Adsense* ad shown increases.
No matter how many referrals the urbanread referral system is built to handle and drive earnings from every referrals you made.
For example if you have made 10 referrals and each referrals gets 100 impressions and that accounts to 200 impressions for your Adsense* ads.
*Adsense is the property of Google Inc.