This is really an odd one. The original, unpublished cover
was drawn by Gil Kane and as you can see, it featured the new character of Xorak
the Outcast, who was introduced in this issue. He was shown battling
Marvel Girl and Cyclops, with heads of the three other X-Men looking on.
For the published version, the art staff replaced the Outcast with the
Juggernaut, who was also featured in this issue, replaced Marvel Girl with
Iceman, replaced Cyclops with The Angel, and changed the onlooking heads.
No one seems to recall exactly why the change was made. The theory has
been advanced that the Comics Code thought the Outcast looked too scary...but
that would have been a reason to tone the drawing down, not to change the
depicted character. It's more likely that Stan Lee (or someone) thought
the Juggernaut was a better "sell" but that's also not a very convincing theory.
So your guess is as good as mine...