Actually, those sound like legit renewal rates to me (I used to be in multiple departments here.) One sure-fire way to tell is to look at the return envelope: if it has a return address of PO Box 3780, Chico, CA 95927, it's really us.
This is common practice in the industry to offer lower rates to first-time subscribers. However, please do give our Customer Service team a call at 800-284-3226 (M-F, 8-5 Pacific). One of the offers they have is the Automatic Renewal program which automatically renews your subscription at the lowest offered rate every year (I think it's $11.88/year at the moment).