Independent is just someone who isn't affiliated with a political party.
Everyone, literally everyone, is somewhere between left and right.
There is nothing outside of that spectrum.
Think of it like average grading scores, on a scale of 0 to 10, with each extreme representing a side of the spectrum (I won't assign the 0/10 to any side since people take offense to even that, but it could be left or right).
You can be a centrist with a fairly evenly distributed opinion range (lean a little bit to each side), like someone whose average grades float between 4 and 6, with an average of 5 of very close to it.
But you could also be an extreme centrist, with opinions that wildly vary in all subjects, corresponding to scores going from 1-2 to 8-9, but still averaging around 5.
Though the average is the same, the standard deviation is much higher.