:ls opens buffer list in a different window
:ls in vim opens a temporary view with the buffer list, and prompts for a command (which will usually be switching to a buffer or deleting a buffer).
:ls is implemented in evil with list-buffers, which opens the buffer list in a split window if there is only one window, or in the other window if there is one. Subsequently, choosing a buffer in the buffer list opens the buffer in the other window, not the window that was active when :ls was invoked.
It would be more faithful to vim behavior if buffer-menu was used instead.