The number of bugs is inversely proportional to the scale of expected usage of a tool.

Imagine each time someone runs into a given bug, there's some chance they report the bug or fix it.

If there are billions of users swarming over a piece of software, the chance that any given bug has been found and fixed is much higher.

This means that software that is single use might have more bugs.

Some bugs will be obvious, but some bugs will be less obvious, especially to non-programmers.