The bacteria's growth over time can be graphed like the red line on this graph:

This is called "exponential" growth. Many people have trouble understanding exponential growth, because we're used to things growing "linearly" -- the same amount from one day to the next, like hair or grass or fingernails.

We expect the bottle to fill with bacteria as if we were filling it with water from a faucet. But any time living things are allowed to reproduce freely, their numbers increase exponentially, not linearly. And when people talk about "steady growth," they mean exponential growth then, too.