there are varioius calculations at play - the level is calculated as the parentlevel +1 , the descendant (child) is then calculated as it's previous descendent +1, and then to give "space" for tabs to move individual descendants within a parent have a (siblingcount*2+1) applied to it i.e. if a parent is at level 1, then children start at 2 * (siblingcount*2+1) -so in the case of a parent with 5 children a new tab will have 2*(5*2+1=11)  =22. However if the tab is added to the bottom it will then have another levy of 3 so a final level of 25 -Note: my calculations may be a bit off but the important thing is that the level differs and not by how much really.

In 6.2.0 all this logic is removed as we dynamically calculate the level using a sql common table expression, so the levels will look more "correctly" ordered AFAIK