If you take the double bond as your starting point and draw the CH3 and H on either side you will see both CH3's could either be the 'bottom' bonded group, either be both the 'top' bonded group or be opposites with one being on 'top' and one being on the 'bottom'.
In an E stereoisomer the groups are on opposite sides whereas with the Z they are on the same. It was far easier when they were called cis and trans.