Positions are relative to what you selected in the step.

They are not necessarily the sibling the position in the input.

For example, given:

<top>
<a/><b/><a/><b/>
</top>

the expressions:

top/a[1]
top/a[2]
top/b[1]
top/b[2]

select all the element children of 'top'. These do not:

top/a[1]
top/b[2]
top/a[3]
top/b[4]