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The gap is to force a line break on long URLs so that the result isn't forced down the page to appear under the last of the right-hand adverts.
Sometimes, very rarely, you will see a VERY big gap after the first or second result with the next result a very long way down the page, and placed directly after the adverts. That occurs on a very very long URL, because Google does not put in any other breaks on a very very long URL, they only have the one break.