If a VC does twenty $10MM investments, and sees a $100MM return on two of them, they break even. No carry would be charged to LP's (although management fees would apply).
Meanwhile an "LP" auto-backing an AngelList syndicator pays a carry on every winner, but still loses 100% for every loser.
That's a big deal in angel investing — generally you're counting on very few winners to cover the costs of your losers. Shaving 15-20% off every winner can very possibly turn your overall returns negative.
Finally, the syndicator has a mis-aligned incentive to broker as many deals as possible. It's now possible to net a profit via carry, even if your losers would have outweighed your winners investing alone.
Tim has fantastic taste in startups. His investment in any startup is a strong signal. But AngelList's terms are pretty iffy for syndicate backers. If the carry was applied to his deals' collective return, I'd be the first in line.