That's easy to answer. It's when the card was graded. I'd venture to guess your 5.5 was graded more recently than the 6 you link to on eBay. My whole issue with TPG, especially with SGC over the past couple of years, has been the change in grading standards.|
If I were you, I'd buy the 6 and send both in to SGC and ask for a regrade. At a minimum, they should switch the grades.
#1 in 1959 Topps