Receiver display swaps between Dolby Digital and PLII - why?

[Vonhelmet]

I have a Humax HD-R Fox T2 hooked up to a Yamaha RX-V471 receiver. When I'm watching HD channels the receiver display alternates regularly, every 5 seconds or so, between Dolby Digital and DDPLII Movie. The sound doesn't seem to vary. What does it mean?

 

[413x]

They are 2 different processing modes for converting sound to 5.1. There is a sound difference with mine. They should only change if told to do so by the remote (as far as I am aware)