Question: why does "extremist" (and flavor) seem to be such a relative term? Isn't it a clear concept within the political spectrum?
The concept of a political spectrum is not, in and of itself, clear.
Take Nazism. To many, it is clear that Nazis were right wing lunatics, yet to me it is equally clear that they were left wing. Their belief in class warfare, with the Jews cast as the evil ones, is vintage left wing in many ways. Try replacing the Jews with the rich in a Nazi speech or replacing the rich with the Jews in a Bernie Sanders speech. Their use of ethnic identification of class has gone out of fashion in the modern left, but it would have seemed perfectly reasonable to liberal racists of the early twentieth century, e.g. Woodrow Wilson.
Left vs. right wing was originally a French thing. At the time, it talked about how politicians chose to sort themselves. The royalists would sit in the right wing of the chamber while the anti-royalists sat in the left wing. One couldn't be extreme right wing at that time. There was no way of saying that one really supported the king's right to be king. Strong support for the king was the base position. It could be moderated but not made more radical.
Does Marine Le Pen support the restoration of the French monarchy? If not, then she doesn't fit the historical use of the term.
One could make an intellectual argument that Le Pen is left wing. After all, she disrupts the government position. That's not the status quo position. If France still had a king and that king supported increased immigration, she might sit in the left wing in opposition.
I've heard extreme right wing used to describe people like Le Pen or Donald Trump who favor government intervention in the economy to prevent trade. And I've heard it used to describe people like me who believe in laissez faire trade. Those are opposite positions!
The truth is that politics is more complicated than can be represented by a single line with a left and a right. Politics is more complicated than a plane that adds a top and bottom. Many positions don't actually contradict each other, and even of those that do, people may still hold both.
The nature of politics is that there is an incumbent position and an opposition position. Originally, right wing was the incumbent position (by definition). Now, either right or left can be the incumbent position. Worse, the actual incumbent determines what is what. So when Barack Obama was president, the left wing (incumbent) position was that free trade was good and opposition was right wing. But when George W. Bush was president, the right wing (incumbent) position was that free trade was good and opposition was left wing.
Since Obama was already left wing and there are right wing supporters of free trade, it's natural that trade opponents like Sanders would be seen as extreme left wing. Obviously it was the less moderate position. But if you flip that around, it's less obvious in the right wing. From one perspective, the idea that economic actions should be decided by individuals and not governments is the extreme right wing idea. Interventionist trade policy would be the moderate alternative.
How can a moderate Republican position and an extreme Democrat position be the same policy?
Politics is not a simple spectrum or even a compass. People's political positions are the result of a large number of decisions, not a simple, left vs. right.