This difference is pretty obvious:

BufEnter X:

It was bufnr() != X

But now it becomes bufnr() == X

BufWinEnter X:

It was len(win_findbuf(X)) == 0

But now it becomes len(win_findbuf(X)) == 1

This difference is pretty obvious:

• BufEnter X:

1. It was bufnr() != X
2. But now it becomes bufnr() == X

• BufWinEnter X:

1. It was len(win_findbuf(X)) == 0
2. But now it becomes len(win_findbuf(X)) == 1