Is Not the Assignment Always Executed in Ruby?

I’m reading the pickaxe book to increase my command of the Ruby programming language. Now I came accross a point in the book that confuses me and I would like to confirm whether I am understanding correctly.

In the section Loops and Iterators > Variable Scope, Loops and Blocks of Chapter 9, the authors explain the scoping rules for blocks such as those used by loop and each saying

Normally the local variables created in these blokcs aren’t accessible outside the block; […] But, if at the time the block executes a local variable already exists with the same name as that of a variable in the block, the existing local varible will be used in the block. So its value will be available after the block finishes.

This is the example from the book to illustrate:

x = "initial value"
y = "another value"
[1,2,3].each do |x|
  y = x + 1
end
[x, y] # => ["initial value", 4]

Here, updating the value of y in the block is reflected outside of the scope of the block because y existed before the block. This does not apply to the block’s parameter. Okay, I get that. But they continue to say

Note that the assignment to the outer variable doesn’t have to be executed. The Ruby interpreter just needs to see that the variable exists on the left side of an assignment.

Checkout this example from the book

a = "never used" if false
[99].each do |i|
  a = i 
end
a # = > 99

Of course, as we can expect a equals 99 after the block gets executed. But at the reason they give, my understanding is not following

The a = i statement in the block sets the outer value of a even though a = "never used" isn’t executed.

My question really one of semantics, because precision is important in programming especially when you are learning. It will be better formulated by me explaining my understanding of the reason why setting a = i in the block is reflected outside of its scope.

Yes, a = "never used" is never executed, but because in that snippet there’s no such an assignment. What is there is a = "never executed" if false, and that I think is executed isn’t it? It is just that the expression "never executed" if false evaluates to nil and nil is the value of a before the block.

Here is an irb session to demonstrate

irb(main):001> a
(irb):1:in `<main>': undefined local variable or method `a' for main:Object (NameError)
irb(main):002> "never used" if false
=> nil
irb(main):003> a = "never used" if false
=> nil
irb(main):004> a
=> nil
irb(main):005>

Put like this I see no special case here. It’s consitent with the fact that in Ruby like in most programming languages when you say a = y it’s a = y.

So, what am I missing?

By the way, the book is a fanstatic read. I would recommend it to anyone who aspires to confidently use Ruby.