No, that's likely not it. The witnessed signature version of most books, especially keys, usually go for significantly more than their non-SS counterparts. GPA will quickly confirm that with few exceptions. For example, sticking with IH#1 and looking at next grade down, a 4.5 SS sold for $18500 in June 2018 (!), when the two 4.5s sold that year sold for only $14.4K and $14850. And a 5.5 SS sold for $20K in March 2016, when the three sales of non-SS 5.5 that year were for only $15,535, $17,860, and $16,200. I only have a couple of biggies (high-grade, too) signed by Stan, and I like them a lot. But I hear you, and being conservative with respect to covers is certainly sensible. Good thing there's something for everyone in this hobby.
You also have to take into consideration that there are not a lot of SS silver age books vs universal and restored books out there so sometimes it's hard to determine a price. It's like a dart throw on how much a buyer wants to pay. I'm just looking at Hulk 1 GPA right now and saw that a CGC 6.0 signed by Stan Lee sold for $22,800 in 2018. A universal graded 6.0 sold that same year for $25k and another for $27,500. Also a 5.0 SS sold earlier this year (Feb.) for $21,550 and a universal one sold on Jan. for $20,350 (not much of a difference). It's a very small sample size due in part to the SS version being so much less out there, but based on those data, the price difference is not significantly more as you mentioned.