Why are AES keys increments of 64-bits? Rijndael, which subsequently became AES, allowed key increments in 32-bits (128, 160, 192, 224, and 256 bits). However, AES made it 128, 192, or 256.
My guess would be that a 64-bit default increment may be good to increase the lifetime of a particular key size for a particular application. For example, assume that an application is using 128-bits and then the application owners feel they have to increase key size because of threats. An increase to 192-bits (cf. 160-bits) may give this upgrade a longer life before technology advancement catches up and the application owners have to change again. Is this speculation in the right direction?