understanding gödel's 1931 paper - primitive recursive functions - “composition” and “insertion”

I am trying to understand Gödel's first incompleteness theorem from his original 1931 paper.

Here is the translations i am using for my studies :

http://mrob.com/pub/math/goedel.html (which seems to be an online version of a 1962 translation)

I am trying to make sense of Gödel's paper so i could explain establish the result from scrath and make it understandable to basically anyone.

I am working on the definition of primitive recursive functions. I am trying to make the precise link between what is described by gödel as "substitution" and what seems to be usually defined as "Composition".

"Substitution" (in Gödel's paper translation) : " A number-theoretic function φ is called recursive, if there exists a finite series of number-theoretic functions $\ φ_1, φ_2, ... φ_n$ which ends in φ and has the property that every function $\ φ_k$ of the series is either recursively defined [180]by two of the earlier ones, or is derived from any of the earlier ones by substitution,$\ ^{27}$ or, finally, is a constant or the successor function x+1. [...] $\ ^{27}$More precisely, by substitution of certain of the foregoing functions in the empty places of the preceding, e.g. $\ φ_k(x_1,x_2) = φ_p[φ_q(x_1,x_2),φ_r(x_2)] (p, q, r < k).$ Not all the variables on the left-hand side must also occur on the right

And "Composition", as it seems to be usually defined : "If f is p.r and has arity m and each $\ g_i$ is p.r and has arity k ≥ 0 then $\ C(f, g_1 , . . . , g_m )$ denotes the unique k-ary function h such that for each k-ary x: $\\ h(x) = f (g_1(x), ..., g_m(x)). "$

My question is then : I guess "substitution" and composition are the same, but why in gödel's paper is "substitution" defined as : $\ φ_k(x_1,x_2) = φ_p[φ_q(x_1,x_2),φ_r(x_2)]$ and not $\ φ_k(x_1,x_2) = φ_p[φ_q(x_1,x_2),φ_r(x_1, x_2)]$ . Is there a reason for that ? Is it to make sure the function terminates (like $\ x_1$ in his definition of primitive recursion) ? Or is it just to illustrate that not all variables on the left hand side have to be on the right hand side ? Or is there another reason ?