I think it's like that on purpose for several reasons. One being that when riding, turns will cause the bet to change it's position on the pulley. If it fit right up to the flanges on both sides the edges would be wearing extremely fast. The problem here is the wheel alignment doesn't jive with pulley alignment, at least on some of the bikes. Ideally the belt would have a tiny gap at the outer flange, but that doesn't happen when the wheel is properly aligned. Instead it is riding hard against the flange causing a lot of wear. You can tell because the wheel gets loaded with belt dust and even shreds of rubber will come off. So like i said, you just turn the left adjuster 2 turns or so so that the axl moves towards the engine a bit. That then buts the wheel out of alignment but takes the pressure off the belt/flange. But i don't feel thats right because if the wheel isn't perfectly aligned then the belt and pulley relationship isn't either. It keeps the belt edge from riding hard against the flange, but it upsets the alignment of the belt teeth to the pulley grooves they ride in. I feel thatss not right either, but if i have to take one of the two i prefer that over the edge of the belt wearing hard against the flange. anyways, thats why i feel it's a flawed system, but probably only because the wheel or pulley needs to go more right. spacers between the pulley and the hug i bet would do the trick. And if so, the triumph would probably eventually rectify it by changing the thickness of the pulley's hub area.
If i'm off here i'd be happy to hear why because i hope i am. But if i am i also hope theres a way to adjust it so this issue doesn't exist somehow, tho i just can't see how.