The 1305-E and 71804/7 are from two different episodes, correct? That's more than enough time to have the -E destroyed or retired and a new ship replace it.
They were ten episodes apart. If there's roughly one week between eps, then that's two and a half months between the (ghost) ship we see in "Where Silence Has Lease" and the ship in "Contagion," which was already on a mission and not just being commissioned. Can a Galaxy class starship be built in less than 2 1/2 months?