Think of Start Fund is like a blind startup index fund. But now that Andreessen Horowitz has made a significant investment in IFTTT I'd imagine they're unlikely to make any future investment in Zapier unless their business changes quite a bit.
I imagine that investing in both by virtue of Start Fund is considerably different from leading a round for both (as in the case of Picplz and Instagram). There's a much higher level of involvement when you're a company's lead investor.
