What you are getting is most likely dependent upon the settings you have selected on the receiver. But it also depends on how you have the PS3 connected to it. To get DTS-HD7.1, you need to be using the HDMI connection, not the optical. And then you have to set the receiver to play what is input into it, rather than some other processing mode.
Just a guess on my part, but I think you probably either are using the optical connection, or you have some processor setting wrong, or both.