OK, so that's a good explanation as far as the head counting is concerned. But the extra head has a body. Well 3/4 of a body: the Siamese twin has his own torso, two arms and one leg. Where is that from?

the 3/4 of a body come from to produce the Siamese twin, who has his own torso, two arms and a leg?

This is kind of like, imagine we have a slide rule with two moving parts:

    . . , | | | |
    | | | | ; ' ' 

Here I have six vertical features, placed side by side. Now I slide the rows relative to each other:

      . . , | | | |
    | | | | ; ' ' 

wee, now I have seven vertical features.

Let's do it with circles:

https://i.imgur.com/9tuNkbT.png

Just by chopping and sliding, I turned 10 circles into 11. Apparently. But of course, the 11 objects are not really circles; they don't have the full area.