UI Autocomplete combobox

UI Autocomplete combobox

I have a form that uses the autocomplete combobox for 3 different selects.  Two of the selects look good but for one the input and button do not exactly line up - the input is 1or 2 pixels taller than the button.  This happens in both FireFox 3.6x and IE 8.  You can see the behavior on the combobox demo page, http://jqueryui.com/demos/autocomplete/#combobox.  Any ideas why this happens and if there is a way to at least have them display consistently.