It is because Perception basically IS Wisdom and Proficiency modifier by itself.
You don't level skills in 5e, there is no some sort of unique Perception score that is not actually a combination of Wisdom + Proficiency (unless given by item or spell explicitly, as in +1 Perception item)
The way they make calculation is 3.5 style, which is a bit silly, but not wrong.
The way it should read is: Skill Check: 11 (1d20) +2 (Proficiency) + 2 (Wisdom) = 15... OR Skill Check: 11 (1d20) +4 (Perception) = 15. I imagine they are not quite sure yet how to write it in such sense that it makes mathematical sense and both tells you that Perception based on WIS+Prof is in play, so they do this. Silly, but not wrong in the end.