With the generic characters when you try to customize best in slot to not include specific gear level or specific dungeons or bosses, or exclude high level enchantments the results will still include those options.
Yeah, it is always going to include the items that were originally equipped on those characters (which are the BiS items for that character at heroic raid level). It is a limitation right now of those “generic” characters.
If you want to look at lower level stuff, one way you can do it is load some random player of the spec you want to look at. Or even a level 1 that you make and then edit the player to be max level with heart of azeroth higher, etc.