help me understand x-over hz?

[liquidice]

so I re-ran my audyssey calibration last night, and got thinking about the settings as them came back after all 8 positions where recorded. It set my mains both to 150 hz, and my rear surround to 150 hz, but put my center channel (exact same speaker as my mains) to 90 hz. Now, the sub of course is at 80 hz as most usually are. Can someone put into simple terms what this all means? From what I know, the sub should only be putting out frequencies below 80 hz, but if the mains for instance are set at 150 hz, where does that freq between 150 and 80 go?...and why is my center channel at 90?...for better quality dialogue? Like I said, I just need an explanation to help me better understand. Thanks.