Uh.. it does not? Alt+Tab on windows switches all the windows, indiscriminately. Cmd+Tab on osx only switches between apps? If you have two Firefox windows on osx, you need Cmd+~ to switch between them, when you have finally reached the Firefox app through Cmd+Tabbing
I am aware that Microsoft and Apple have implemented fast window-switching slightly differently. I use both OSes daily. The point I was making was both make similar high-level design decisions.
When you begin alt-tabbing, you cycle between your other open windows, arranged by how recently they were last open. Cmd-Tab is the same, but between apps. Both features rearrange items only based on recency, and always keep the same order.