- and can not be slipped as the Buntline hitch -
What am I missing? Plainly it can be slipped since the tail is readily available and could possibly lead to easier untying.
Good question !

I know that most knot tyers think of the "slipped" variations of knots as just convenient ways to facilitate the untying of them, or the release, with one pull, of knots that may be still under tension.
However, those "slipped" "variations" are
different knots, in three ways. Firstly, a slipped "variation" of a knot can sometimes be TIB, while its non-slipped parent knot may be not. That means that those two knots may be
topologically different, i.e., as different, as knots, as they can be !

Topology is the most important characteristic of the mathematical knots, and I do not see why it should be of secondary importance in the filed of ideal or even "real", physical knots.
Secondly, the "added" new segment, the extension of the previous tail, changes the geometry of the knot, by its mere presence, by the bulk of its material that forces the segments of the parent knot to re-arrange themselves, follow different paths inside the knot s nub, be loaded differently, change the distribution of tensile and sheer forces within the whole knot ,etc. So, they are important not only topologically, but geometrically as well.
Now, last but not least : The extension of the tail which is used in the "slipped" "variation" can function in a new way, as a new "obstacle" that would block the slippage of other segments, and contribute to the overall stability of the knot. That is exactly what the extension of the tail that retraces its last part does in the case of the Buntline hitch ! It transforms it in
an entirely new, different knot, more complex, but also
much more effective. The inserted segment meets the penetrating main line at a right angle, at
a fourth point, so we can say that it is added on top of the other three obstacles that this main line has to overcome, in order to slip through the nub of the hitch. So, according to my understanding, the "slipped Buntline" hitch
is a different knot, that, roughly speaking, is about
four thirds as efficient as the Buntline hitch - 33% more ! ( In the Buntline hitch, the main line is blocked, by an "obstacle" placed at a right angle to it in
three points, the two collars and the initial tail, in the "slipped Buntline" it is blocked in
four ).
I do not see how this can be achieved in the case of a "Slipped Figure 1" - that is why I said that the Figure 1 knot can not be compared to the Buntline hitch, regarding their "slipped" variations.