The Brand Matters
Not all Site Seals are created equal—just like not all Certificate Authorities are created equal. Within the realm of web security, some companies have better standing than others. A static RapidSSL Site Seal isn't going to offer as much assurance as a dynamic Comodo Seal. And all of them pale in comparison to Symantec.
This is the result of years of higher visibility and greater market presence. When a visitor sees that Norton checkmark they are going to recognize it a greater percentage of the time and, in turn, feel safer. This is one of the results of Symantec being the most well-known, well-respected name in the web security industry.