How to explain the result of executing the code (element by element)?

Question

ruby

There is an initial array: There is a code:
a = [1, 1, 1, 1, 1, 2, 2, 2, 5, 5, 6]

a.each do |item|
  a[item] += 1
end

Execution result: I seem to understand what the code above does, but I can’t explain the origin of the elements except for the very first zero a[0]
a = [1, 4, 6, 1, 1, 4, 3, 2, 5, 5, 6]

Answer 1

a = [1, 1, 1, 1, 1, 2, 2, 2, 5, 5, 6]
There are 11 iterations in total.
#1

a[1] += 1
#a = [1, 2, 1, 1, 1, 2, 2, 2, 5, 5, 6]

#2

a[2] += 1
#a = [1, 2, 2, 1, 1, 2, 2, 2, 5, 5, 6]

No. 3

a[2] += 1
#a = [1, 2, 3, 1, 1, 2, 2, 2, 5, 5, 6]

#4

a[1] += 1
#a = [1, 3, 3, 1, 1, 2, 2, 2, 5, 5, 6]

#5

a[1] += 1
#a = [1, 4, 3, 1, 1, 2, 2, 2, 5, 5, 6]

#6

a[2] += 1
#a = [1, 4, 4, 1, 1, 2, 2, 2, 5, 5, 6]

etc.