If it wasn't for us being biased by the surrounding circumstances I don't think people would have confused their voices. Their voices are not that similar. I probably personally know people with a voice as similar to SJ as Sky's. You probably do too.
The voice actress says the same: "I’ve never been compared to her by the people who do know me closely."
But then suddenly a story emerges and their voices are indistinguishable. All of these extra details shouldn't have even mattered.