I will have to look into that. And my husband isn't just my sempai with swords, sensei considers him my sempai...period. I guess I just won't get it and I think asking how it works would be rude.
Your sensei is importing a culturally specific social convention into a foreign environment. It is not inappropriate to ask him to explain it, or at least his interpretation of it, to you in detail. In fact, it's his responsibility to do so. To not do so invites, well, loss and confusion.
Your sensei may have a misunderstanding about sempai-kohai. Or maybe he understands quite well but has not adequately explained his understanding to you. As Professor Goldsbury suggests, he might be wrong. OTOH, it's his dojo to order as he pleases. Or maybe he simply has the mistaken impression that your husband started earlier than you. In any event, he is bound to give more useful
answers than we can here. I strongly recommend you broach the matter with him directly, and I doubt it will be a problem. If it is a problem, perhaps you should reconsider your place of training.