The problem is that Mobipocket moves backwards and forwards in 64k blocks. So if you jump to a specific chanpter, it's going to go frward and back from there in 64k blocks. Now if you go to start reading, it's gojng to go backwards and forward in 64k blocks.
Jump to chapter 5. Then go to page 20
Jump to start reading and go to page 20
You will be on a different part of the book.