How are Early Bird boarding positions assigned?

Curious traveler here, not ranting - purely just curious. So please don't think I'm attacking someone with my question or that I am complaining.

I bought my ticket 6 months in advance and added EB. Then, I purchased a ticket for someone 2 months ago with EB.

The other person got an earlier boarding position.

I've always been curious as to how boarding positions are assigned. Does anyone know how this works?

Truly appreciate you all!