Why does 1 object sink while a nearly identical object – just a fraction of an ounce lighter – floats effortlessly to the top?

Because there's a critical threshold above which you're expanding and below which you're losing ground. There needn't be any qualitative difference between the 2 outcomes. It may be simply be a small difference – a bit more of the same stuff.

That's why it's important to work hard on every aspect of your project – because neglecting any area might cost you your buoyancy.

Think about everything – content quality, types of media, publication pacing, social sharing, email list segmentation, return visit rates, user interface design, cross-marketing, SEM, link building, branding, yadda yadda yadda.