Because if a rail is bowed up at the ends, then bolted down to the slate, it'll cause the slate somewhat to change level as the rail can have a tendency to cause the slate to change level under the stress of trying to keep the rail flat to the slate. So to really level a table, one really needs to dry level the slates with the rails bolted down first with no cloth on the slate. Then adjust the level of the slates to counter act the stress of the rails being bolted down to them. Keep in mind, once the rails are taken off to install the bed cloth, the slates are going to read unlevel, until the rails are remounted again.
Glen