Some "mixed" chips have more than one reference voltage, which is most often used for output (drivers). In some kind this has to do with "output", too, as the analog output should mirror the input.....
The 405x are special in that they are made to allow NEGATIVE input, as their main application is audio, which is AC. To avoid the need for adding a constant DC to it, the 405x have a second analog reference, call Vee.You can set this pin to a negative voltage downto -5 V, so you can get a total voltage swing of 5 - -5 = 10V. In your application however, Vee shoud be connected to ground.
OT: There is one little known exception from the general rule: 4049 and 4050. Those drivers/inverters are specifically made to be tolerant to a higher input voltage than their Vcc. This is why you find them sometimes as a protection between microcontroller inputs and the real world.