Thomas is correct (as usual), but there are also other, non-technical reasons for using particular 'grades' of encryption.
At certain classifications you are obligated to use a particular type and key bit-length. The reason for it is providing assurance that the data is going to be protected for at least 30yrs instead of 10yrs. Yes, there will be attacks on algorithms, and they will diminish the level of effort needed to bruteforce it. While 2^167 and 2^99.5 bit keys both fall squarely in the 'ludicrous' territory, given enough time, Moore's law, and possibly even more/better attacks further reducing the effective keylength, we might get to the point in near future where bruteforcing AES-128 might be possible, but 256 variety might still be way out of anyone's computational ability.
That's the bet you're making with going for longer key bit-lengths: it might be breakable in 30yrs but not in 10yrs, and by then, the information protected won't carry as much impact if decrypted by the attackers.