Why are the session keys in DHKE for different values of 'a' the same?
Since $\gcd(2,467)=1$, one can observe using Fermat's little theorem  that $4^{233}\pmod{467}=2^{466}\pmod{467}=1$.

Thus,
$$g^{a_1}=4^{400}=4^{233+167}\equiv\underbrace{1\cdot4^{167}}_{=g^{a_2}}\pmod{467}=89.$$

This results into identical session keys (shared secrets) $S=(g^a)^b=89^{134}\equiv161\pmod{467}$.