HoMM VII Haven Units – Haven faction units from the seventh installment of Heroes of Might and Magic.

What exactly makes it more difficult to obtain the "Guardsman" model than the "Sentinel" model?

It would take a long time to explain. It's easier for you to just accept it.)

Added 19 minutes later
In short, you can say that a 3D character consists of:
1. The model itself (a three-dimensional figure with a texture, which is a two-dimensional image, applied to its surface).
2. A skeleton (a set of control elements, "bones," with which various parts of the model move).
Skinning (or binding) is the connection between specific bones and specific parts of the model (i.e., so that when you move the bone responsible for the arm, the arm of the model moves, and so on).
3. Animation (a recorded sequence of bone movements; when played, it produces a specific movement, i.e., a creature "runs" because the shin bone rotates relative to the thigh bone by a certain degree, and so on. All of this is recorded in an animation file and is now being played.

Now, the answer to the question, "Why is it more difficult to get a model from Heroes of Might and Magic 6 than from Heroes of Might and Magic 7?"
From Heroes of Might and Magic 7, you can easily extract the model, the skeleton, and the animation. From Heroes of Might and Magic 6, you can only extract the model and the skeleton (and the skeleton is often flawed, and the skinning is broken).
The skeleton from Heroes of Might and Magic 6 is not compatible with the animations from Heroes of Might and Magic 7. Therefore, it is necessary to either re-do the skinning (bind the model from Heroes of Might and Magic 6 to the animation from Heroes of Might and Magic 7) or perform other complicated procedures. In any case, this is much more difficult than simply taking a model from Heroes of Might and Magic 7.