Why do they use 'winning by anywhere'?

Question

Why do they use 'winning by anywhere' not 'winning everywhere' what is more clear for non native speaker?

Exit polls have Jim Matthews winning by anywhere... from a nine to 12 percent margin of victory.

Answer 1

You are parsing the sentence incorrectly.

It is not:

(winning by anywhere) (from X to Y)

But:

(winning) (by anywhere from X to Y)

In other words:

Exit polls have Jim Matthews winning [and the votes for him exceed those for the others] by anywhere from a nine to 12 percent margin of victory.

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Further, anywhere from is not being used to describe a place but a number range.

These two expressions are functionally equivalent:

anywhere from a nine to 12 percent margin
a nine to 12 percent margin

The use of anywhere from only serves to provide emphasis.

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Taking both into account, you could shorten the entire sentence into something like this:

Exit polls have Jim Matthews winning by a 9–12 percent margin.

Answer 2

The expression 'anywhere from [some quantity or number] to [some other quantity or number]' means 'an unspecified, unknown, or unstated quantity between a lower limit and an upper limit'. Since each such quantity is a single one, 'anywhere' is used, and not 'everywhere'. Repairing your TV could cost anywhere between $50 and $200, depending on what went wrong.