1: [-3 .. 3] 2: [-3 .. 3] 2: [0 .. 9]
. 1: [0 .. 9] 1: [-9 .. 9]
. .
[ 3 n .. 3 ] RET 2 ^ TAB RET *
In the first case the result says, "if a number is between -3 and 3, its square is between 0 and 9." The second case says, "the product of two numbers each between -3 and 3 is between -9 and 9."
An interval form is not a number; it is a symbol that can stand for many different numbers. Two identical-looking interval forms can stand for different numbers.
The same issue arises when you try to square an error form.