Small padding, big difference

Question

If I only add a 1px padding to a div around a heading, then this makes apparently a huge difference (http://jsfiddle.net/68LgP/).

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Answer 1

It is now keeping the margin of the h1 within the DIV. The h1 has default top and bottom margin of around 21px, so when you add 1px padding to the DIV, it now looks like 22px