I don't understand completelly what you are trying to create or what your goal is, but seems to me that beacuse "slab based family" has different default settings than project, it just shows different parameters.
In project, you insert a famaily (for instance Generic Model) and it shows you the "Level" to which you want to insert it to.
When you operate with Slabs (Beams) or Walls, the default setting is that you have Base Level/Offset and Top level/Offset.