Goodman had like a 3 month period to try other cover artists before Jack took over completely.
Kirby Covers.
September 1958 - 3 of the 4 Sci-Fi covers
October 1958 - 0 of the 2 Sci-Fi covers
November 1958 - 0 of the 4 Sci-Fi covers
December 1958 - 1 of the 2 Sci-Fi covers
January 1958 - 3 of the 4 Sci-Fi covers
February 1958 - 2 of the 2 Sci-Fi covers
March 1958 - 4 for 4 of the Sci-Fi covers
April 1958 - 2 for 2 of the Sci-Fi covers
April 1958 - 3 for 3 of the Sci-Fi covers
ETC
No. Kirby's stories led off 90% of those books. MORE than 90%. FACT. You're just repeating what you've been led to believe.
I'll show you one, and then you can go do the rest on your own:
30 issues of Journey Into Mystery from #52 to #82 (all PRE-Thor)
Kirby - 28 times with the lead story.
Ditko - 2
93.3% (That's MORE than 90% of the time)
Facts.
Here's what I've read thus far. This covers the period from 1958-59. For clarity's sake, the lead feature is defined as whatever appears on the cover regardless of the order in which it appears.
Strange Worlds #1 (Cover: Kirby, LF: Kirby)
Strange Worlds #2 (Cover: Ditko, LF: Heck)
Strange Worlds #3 (Cover: Kirby, LF: Kirby)
Strange Worlds #4 (Cover: Kirby, LF: Buscema)
Strange Worlds #5 (Cover: Kirby, LF: Heck)
Tales to Astonish #1 (Cover: Kirby, LF: Kirby)
Tales to Astonish #2 (Cover: Ditko, LF: Sinnott)
Tales to Astonish #3 (Cover: Kirby, LF: Reinman)
Tales to Astonish #4 (Cover: Kirby, LF: Sinnott)
Tales to Astonish #5 (Cover: Kirby, LF: Kirby)
Tales to Astonish #6 (Cover: Kirby, LF; Ditko)
Tales to Astonish #7 (Cover: Kirby, LF: Ditko)
Tales to Astonish #8 (Cover: Kirby, LF: Heck)
Tales to Astonish #9 (Cover: Kirby, LF: Heck)
Tales of Suspense #1 (Cover: Heck, LF: Heck)
Tales of Suspense #2 (Cover: Ditko, LF: Sinnott)
Tales of Suspense #3 (Cover: Buscema, LF: Sinnott)
Tales of Suspense #4 (Cover: Kirby, LF: Ditko)
Tales of Suspense #5 (Cover: Kirby, LF: Forte)
Tales of Suspense #6 (Cover: Kirby, LF: Ditko)
Tales of Suspense #7 (Cover: Kirby, LF: Kirby)
Tales of Suspense #8 (Cover: Kirby, LF: Kirby)
Tales of Suspense #9 (Cover: Kirby, LF: Kirby)
Strange Tales #67 (Cover: Sinnott, LF: Ditko)
Strange Tales #68 (Cover: Kirby, LF: Sinnott)
Strange Tales #69 (Cover: Kirby, LF: Burgos)
Strange Tales #70 (Cover: Kirby, LF: Ditko)
Strange Tales #71 (Cover: Jack Davis, LF: Ditko)
Strange Tales #72 (Cover: Kirby, LF: Kirby)
Strange Tales #73 (Cover; Kirby, LF: Kirby)
Strange Tales #74 (Cover: Kirby, LF: Kirby)
I haven't read Journey into Mystery yet, but let's have a looksie.
Journey into Mystery #49 (Cover: Maneely, LF: Forte) NB: not new stories, but the book returned from hiatus and changed to sci-fi from this issue
Journey into Mystery #50 (Cover: Sinnott, LF; Ditko)
Journey into Mystery #51 (Cover: Heath, LF: Ditko)
Journey into Mystery #52 (Cover: Kirby, LF: Sinnott)
Journey into Mystery #53 (Cover: Kirby, LF: Heck)
Journey into Mystery #54 (Cover: Kirby, LF: Ditko)
Journey into Mystery #55 (Cover: Kirby, LF: Ditko)
Journey into Mystery #56 (Cover: Kirby, LF: Heck)
Journey into Mystery #57 (Cover: Kirby, LF: Heck, Kirby (splash))
I'm not going to bother with the math, but I can tell you it's not 90%. Even if you consider the lead feature to be the first story in the book, it doesn't help Kirby's case much. The fact is the pattern of Kirby doing the cover and the lead feature (as the first story in the book) was established in Tales of Suspense and Strange Tales once Kirby began doing the monster stories not sci-fi stories.