Why was it cheaper to efile Mom's return as guest?

I e-filed my mom's returns (federal & Georgia) today as a guest through Intuit and was only billed $20. I e-filed my own returns over 2 weeks ago (federal & Georgia) while logged into my Intuit account and it was $25. Why the difference? If it is cheaper to e-file as a guest, I will start doing that in the future.