I am pretty sure it has to do with the way the channels first appeard on Dish. Inertia. Originally Dish added Starz and Encore (just the two channels) and charged $9 a month for them. They later added Encore Westerns and Starz 2 (Which changed its name to Starz theater and now Starz Edge). When they added 110° and AT150 (now AT180) they added the rest of the Encore channels and many of the Starz channels to the line-up. But they kept Encore as part of the Starz package. Encore Westerns continues to be available to Staz/Encore subscribers who have not changed their programming since the April 2000 addition of AT180.
So, due to the way they originally packaged the channels--
Starz Package includes Encore East and all the Starz channels on Dish.
AT180 includes Encore west and the 6 thematic channels.
Encore Movie Pack (available for $4.99 added to any AT package includes Encore West and 5 thematic channels (Not WAM).