User blog comment:B214/Volume 17 spoilers./@comment-1359322-20140713130154/@comment-16281343-20140714225133

It's a japanese thing (and also in other languages). You can't notice the difference when it's translated to english.

~There's a different way on how you speak to a person higher in rank than you or older than you (in case of Issei it's both) and to someone in the same level and the same age.