Sorry, I misread that bit in your post.
Strange.
I just checked mine to be sure before posting. I have three consecutive channels, 7, 9, and 10. I selected channel 9 and pressed the channel up (+) button, and it moved to channel 10. I pressed the channel down (-) button and it moved back down to channel 9, then to channel 7. The (+) button is the top button on the remote, the (-) is the bottom button.
On the Guide it does work perfectly as a Page Up/Down, and even stops at the top or bottom of the guide instead of jumping up to the menu. In Theaterview on a list of TV Shows Page Up/Down will move through the list and scroll back to the top if you pass the bottom, or to the bottom if you pass the top, which is nice as well.
So my setup works differently to yours.
On previous software/hardware I have used, the Channel Up/Down buttons moved up and down a selected default favourites list instead of up/down the numeric list of channels, and did behave the way you explain.
There is an option for this in MC, under Options > Television > Advanced > Edit channel list at the top of the dialogue page, called "Next / Previous use favorite channels only". Do you have this selected? If so, maybe it changes the functionality, since now the Channel Up/Down buttons are moving up and down your favourites list which is sorted lowest number at the top, and not the numeric channel list. . .
. . . Hmm, no. I just checked on my setup and changing that parameter didn't not change the Channel Up/Down sequence. It just correctly only moved between favourite channels.
Sorry, no help here. I don't know what is going on.
PS: I was testing this with the channels showing live on the TV, so nothing strange going on for me Yaobing.