Fixtures and pipes ( and other fixtures) not lining up on Y axis

It appears you have the pipe nested within something in the second example. When it is nested, the world value should show the pipe's real world location relative to the model origin while the position shows its position relative to the origin of the object it is nested. When it is not nested, the world value and the position will match as both are relative to the model origin. (As is the case in your fixture example)

Nested devices retain their world position when nested to a parent and the offset is calculated. If you wanted both to be at the same Y value, you would want to have the position of the pipe at the same world Y as the fixture prior to nesting or adjust the Y position to account for the offset of its parent object after nesting the pipe.

To expand on Kirk's answer:

You might want to place a chalice on a table.
Chances are that if you do that, you want the chalice to stay on the table no matter how you move and rotate the table within the scene.

"Position" is the position relative to the immediate parent - the table.
This won't change when you move the table around - so the chalice stays on the table.

"World" is the overall location within the world.
When you move or rotate the table, the chalice is still in the same place on the table - yet has clearly moved in the world because the table used to be stage left and now it's stage right!

You can see and manage these relationships in the Hierarchy.
Simply drag an object onto the right 'parent' object, and it will now move whenever the parent moves.