jQuery Grid Plugin - jqGrid

I've observed that if I change the padding on the below line to a value of 0.8em, it causes width issues for the grid.  Basically, the more columns I have in the grid, the more it will cause horizontal scrolling within the grid (using this padding value).  If I change my padding setting to something in pixels, the problem goes away:

original CSS line in ui.jqgrid.css:

.ui-jqgrid tr.jqgrow td {font-weight: normal; overflow: hidden; white-space: pre; height: 22px;padding: 0 2px 0 2px;border-bottom-width: 1px; border-bottom-color: inherit; border-bottom-style: solid;}

modified CSS line with padding in em units (this has the problem I mentioned):

.ui-jqgrid tr.jqgrow td {font-weight: normal; overflow: hidden; white-space: pre; height: 22px;padding:0.8em;border-bottom-width: 1px; border-bottom-color: inherit; border-bottom-style: solid;}

However, this modified padding setting does not cause the problem:

.ui-jqgrid tr.jqgrow td {font-weight: normal; overflow: hidden; white-space: pre; height: 22px;padding: 10px 10px 10px 10px;border-bottom-width: 1px; border-bottom-color: inherit; border-bottom-style: solid;}