Condition evaluation stored in a variable inside (if statement) gives correct results whereas direct evaluation in (assert) gives erroneous results

Question

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;

entity EqTB is
end EqTB;

architecture Behavioral of EqTB is
    signal a_in, b_in: STD_LOGIC_VECTOR(1 DOWNTO 0);
    signal