They're just different than the "stock" icons. There's a lot of things that can be done with "skins".
The stock Xenforo has very few icons, actually. It's mostly text links. I have visited a few forums where there are very few text links that mostly use icons.
The engine is the same. Just different user interfaces.
Take a look at
The SitePoint Forums for example. Same engine, different UI.
One I kind of like is
The Admin Zone
They're all use the Xenforo software engine.