It’s due to response profiling. For different ISPs, the historical data on offers redeemed is tracked, and the response rate for yahoo users seems to be tied to a higher discount offer. If the offer is stated in the subject line, then the rate is first an inducement to get you to open and read the e-mail. For 40% a yahoo user is more likely to redeem the coupon, or make an online purchase than at a lower rate.