OSX: modifying text selection with ⌘⇧→ and ⌘⇧←
Mac doesn't use the cursor position if you have a multiple text selection, it considers the cursor is at 'all selected text'. 

You can see this demonstrated if you use  Cmd ⌘   Shift ⇧   ←  instead of  → 
Adding to the selection with  Shift ⇧  includes the previous selection.

Not a fix, but a workaround might be  →  which would move the cursor to the end of your current selection, simultaneously deselecting it, then  Cmd ⌘   Shift ⇧   ←   to select to the beginning of the line.