I think it's easier. I imagine there is a set number of spots per boarding group, pretend it's 100. A solo person can fill any spot left in a BG, whereas groups would need to be smaller than the remaining spots left in a BG. I have no clue if it actually works like that, but I got BG 6 (first group was BG 3) and then BG 8 (first group was BG 8) on my two solo mornings in HS recently.