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The line x = list(var('x_%i' % i) for i in (0..1)) defines x to be a list, and no matter what numbers you put in (m..n), lists in Python are indexed starting at 0. For example:
sage: x = list(var('x_%i' % i) for i in (1..2))
sage: x
[x_1, x_2]
sage: x[0]
x_1
sage: x[1]
x_2
sage: x[2]
---------------------------------------------------------------------------
IndexError Traceback (most recent call last)
<ipython-input-7-481b12fa0bac> in <module>
----> 1 x[Integer(2)]
IndexError: list index out of range
That is, x[0] refers to the 0th element of the list x; the part of the command for i in (1..2) doesn't affect the list indexing. You could use a dictionary if you want different indexing:
sage: x = dict((i, var('x_%i' % i)) for i in (1..2))
sage: x # the numbers 1 and 2 are the keys: the allowable indices
{1: x_1, 2: x_2}
sage: x[1]
x_1
sage: x[2]
x_2
