The bidi.css file has a rule which adds a negative margin to tables, this affects the left side of the table surround when using an RTL language as it makes it slightly wider than the side of the table.

There isn't any rule for this in any other file for non RTL languages so it must have been removed at some point in the past and the rule in the bidi.css file forgotten about.

File:

bidi.css

Delete:

.rtl .forumbg table.table1 {

    margin: 0 -1px -1px -2px;

}