This has happened to me as well but it has never been off by more than a day. Best I can figure is they aim for a date that will get them close to when it should be on shelves in the homebrew shop. I mean, they are printing these labels ahead of time, maybe with a specific date to fill as a goal and the yeast might be ready early.
Also "4 months" is not exact. Is 4 months from today 5/5? Or is it 30x4 days. Or 4x4 weeks? Anyway, just use today as the date for the Mr Malty (or maybe like 3 days ago) will get you close enough.