Possibly the resolution is above 720x576/720x480. Or what you have isn't DivX, it could be x264 in Mp4.
However Mp4 is a different container to avi which is why it won't play them if they really are DivX. The philips might support Mp4, I don't know because the specs sheet only says DivX Ultra 6, it doesn't mention the containers supported.
I would suggest that you get a media player because those will play most videos and won't have the severe limitations that the Philips has.
I suggest that you get MediaInfo and check the files to find out what they are.