Acclimation.
When she spent time in private with Hajime there was no issue with his limited understanding of contemporary culture.  When they were alone Kikuya and Hajime understood each other perfectly as her very traditional training matched up with the world he knew.  It was when they were with the others or in public that things became ever so slightly more complicated.  There were simply things that Hajime had no frame of reference for.  Kikuya felt it was her duty to assist in helping him understand.  It would make his daily life easier.  That was important to her.  One afternoon Kikuya decides to correct this and she shows up at the offices with three large shopping bags in tow--dressed in street clothes.