“Why isn't this line formatted correctly, but formatting work(s) here on Meta?”

Question

If you look here, you can see last revision, from Cody Gray, that he changed "this line of code does not work correctly on stackoverflow but works here on" to "this line of code does not work correctly on stackoverflow but work here on"

How did he know that it will be "work" and not "works". Because it's already once before in that sentence?

Answer 1

You haven't transcribed his edit accurately.
His edit:

Why did this line of code: {...} not format correctly here on Stack Overflow, but work correctly here on Meta?

The parallelism is: "did not format" / "but (did) work correctly".
You've quoted him as saying "does not work" / "but (does) work"; unfortunately, for reasons I can't quite express, an implied "does" doesn't sound right here, while an implied "did" does.

However, I believe you're asking why he edited your question in the first place. Essentially, it's because you didn't phrase it properly as a question.
You asked:

Why this line of code {...} did not format correctly?

Generally in English if we want to ask a "why" question we put the verb first, before its subject; the answer will have the subject first (although the verb phrase will often be omitted entirely.)

Q: Why did the chicken cross the road?
A: (The chicken crossed the road) to get to the other side.

Q: Why did this line of code not format correctly?
A: (The line of code didn't format correctly) because the Stack Overflow gods were angry.

There are two cases where "Why this line of code didn't format properly" would be correct; one is as part of an answer:

"A comma was missing; this is why this line of code didn't format properly."

and the other is as a title or headline:

"Why this line of code didn't format properly: a tragedy in one act"

Answer 2

It's much simpler than you would have thought!

First, let's define what we're talking about - a line = 1 line. Therefore:

This line of code works perfectly.
This line of code doesn't work at all.

This is the only way to use them in a sentence. Combine those sentences, and you get:

This line of code usually doesn't work, but works on this site.

EDIT: Now I looked at the original post and it's a different case than you presented.

In your question here, you wrote, "This line of code does not work ..."
In the original post by Cody Gray, he says, "This line of code did not format correctly."

In case of the past tense exactly as Cody Gray wanted it, the following is correct:

Why did this line of code did not work on Stack Exchange, but worked correctly here on Meta?

BUT, as he started the statement with "did" and then followed with all the cases, he basically wrote it as:

(Why did this line of code)
a) (not work on Stack Exchange), but
b) (work correctly here on Meta?)

I believe what he did there is just plain wrong and should be avoided.

It's important to realize what tense(s) you want/need to use in your sentence(s). What you wanted to ask is this:

Why didn't this line of code format my article correctly on Stack Exchange, but works on meta?

Here you used the past tense in the first sentence and the present tense in the second sentence.

As long as it's currently still a problem you are trying to solve, you should probably use the present tense in both sentences, as the problem is still there:

Why doesn't this line of code format my article correctly on Stack Exchange, but works on meta?

Or, you could say it all in the past tense, for whatever reason:

Why didn't this line of code format my article correctly on Stack Exchange, but worked on meta?

Now that you see how many alternatives there are, it's easy to assume that Cody Gray simply mixed up what you actually wanted to say there and broke your sentence.