Hi! Thank you for your reply!
Based on the previous years thread, this is one of the few schools that sends out invites in December/does super early interviews. Not saying that this is what that is, but maybe this department likes to meet and talk as soon as possible with potential students to have an idea of what they're like before they actually view the full application materials.
I sent him my CV and quite a detailed email outlining my experience and interests. Also, this is a child development lab, and I work with children with serious neurodevelopmental disorders. I think maybe it was a combination of my email, CV and experience that prompted him to ask for a Skype meeting.
Honestly, I didn't think it was that unusual for a POI to set up a call/Skype meeting. There are two other profs who emailed me back saying they would like to do a call in late October when they are not so busy... so I thought it was kind of common to talk via Skype before the application deadline?
I'm a bit nervous though because I think my application is strong besides my GRE scores (terrible quant scores, like the worst ever), I'm retaking soon to up my score. Hopefully it goes well! As exciting as it is to receive interest, I'm reminding myself that there are so many other aspects of an application that go into the decision making process besides research fit/experience.
Edit: Thank you for your advice, it was very helpful!