Prestashop buttons shift - a bug?

[Sanis]

Iam wondering, why buttons in prestashop are always slightly displaced in up in down directions, inluding the rollover?
This may not be noticeable using plain buttons, but with drop shadow buttons it certainly is.
Tested with IE8 and Firefox using default prestashop theme.

/* classic buttons */
   width: 97px;
   background-image: url('../img/button-medium.gif');
/* global button */
   line-height: 16px;
   height: 20px;
/* hover button */
background-position: left -20px;