It's not gratuitous. One jug + one dozen = 13 jugs.

jugs = 1; if (have eggs) { jugs = 12; }

Get a dozen, not a dozen more.

  cart.addJugs(1);
  if (haveEggs)
    cart.addJugs(12);

Now the cart has 13 jugs.

I'd say that "get a dozen" implies "jugs = 12" and not "jugs += 12". If you wanted to get a dozen more, you'd say "get a dozen more"

This is why we don't program in English.

And why it is so hard to write a good specification.

"get a dozen" implies "jugs += 12"