Evil Dead II was Sam and Bruce's spiritual "re-imagining" of the original "The Evil Dead" (now with real money!)
In other words, "Now that we have some money, let's make The Evil Dead how we always imaged it!"
The fact that ED1 was such success compared to it's original dollar-store budget attracted many more investors to the concept of a "sequel" to "The Evil Dead."
This afforded Sam & Bruce the financial ability to shoot ED2 as they originally intended. Yet they could never go on the record and say, "Yeah, it's just ED1 but we had catering this time."
And in spite of it not actually being a true "sequel," the fact that it was even more of a financially success than ED1 allowed both to further their careers in Hollywood.
Therefore, Army of Darkness is essentially "Evil Dead 2."
Thank you for coming to my TED Talk.