I assume you have the older PS3, not the PS3 slim and that you're connected via HDMI to the AVR. The older PS3 cannot bistream Dolby TrueHD or DTS-MA to the AVR, so the decoding of these formats must be done in the PS3 and Multi-Channel PCM is then sent to the AVR. This is still lossless audio, with the decoding done in the PS3 and you'll never see Dolby TrueHD or DTS-MA displayed on the AVR from the older PS3.
This is discussed in the following FAQ on AVR features - the HDMI/HD audio section near the end.
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