It adds to the days because under the hood, a datetime is really two integers. The first integer is the number of days since 1/1/1753. The second integer is the number of 0.003 timeslices since midnight. When you add an integer to a date, it adds the number to the first integer, so you increment by N days. To play with this a little more, try this:

SELECT GETDATE() + GETDATE();

For a great list of date routines, see Lynn's post at http://www.sqlservercentral.com/blogs/lynnpettis/2009/03/25/some-common-date-routines/.

Sean is right - the right way to do it is to use DATEADD.