Toggling between flags is a feature of the Digital Safety Flag software called Flag Link. This features links one flag to another so they appear to toggle back and forth between both flags. Once configured, the primary flag will display 60% of the time and the linked flag will display 40% of the time. This is why you notice the PC is displayed longer then the double yellow.
To set it so the double yellow displays longer, make the double yellow the primary flag and link it to the pace car PC flag. Now the double yellow will display 60% of the time and the PC will display 40% of the time. Here are screen shots of the two setups: