HD Premium Channels

Hello, I am new to Comcast and find it very strange my premium channels are not all in HD?  Also the channel lineup is all over the place from single digit, double digit, triple didgit and four digit channels.  Can anyone give me the lowdown on what gives and maybe an easy way to sort out all of these channels?  I checked for a HD add-on so I could get all channles in HD but couldnt find anything, so any information is helpful... thank you