We are experiencing similar problem. For the current shopping cart, inventory availability does not prevent an item being added to the cart when any product attribute for that item is different than the same item that has already been added with a different value for any product attribute.
Product set to 'track inventory'
Product has at least one attribute
Product set to 'No backorders'
Example: Product Availability = 1, product attribute set to "ABC" and add a quantity of 1 to the cart
Now, try and add that same product with product attribute set to "XYZ", it allows the 2nd item to be added to the cart even though line 1 would have consumed inventory available.