Why does Mr. Rochester so often call Jane 'Janet' in the book?

At first I thought it was a typing error, but upon further meditation, it appears too frequently to be put down to a mere mistake - and adele too; there are a couple instances where Mrs. Fairfax refers to her as 'Miss Adela.' Is this an error? o is it a kind of nineteenth century endearment, o nickname of some kind? o have I just completely missed the point?