Are you surprised by how high Shania Twain and Garth Brooks are on these music lists?

No, not at all. First, the Country/Western genre is exceedingly popular (note that 4 of the top 10 artists are Country artists). Second, in the decade we're looking at, both Garth and Shania had HUGE crossover hits, that were popular with lots of people who didn't ordinarily listen to country music.