hardy heron in virtualbox

i discovered something strange about running ubuntu 8.04.1 hardy heron in virtualbox. when i resize the virtualbox window the resolution of ubuntu automagically resizes to whatever size the VB window is. i'm not sure why it's doing this but i'm not complaining. it's pretty cool actually.
notice the strange resolutions, 1576x1103, 1472x1061, 1184x859, 1440x1050.