No offense intended, but when I refer to "external fragmentation", I refer to fragmentation outside of SQL Server's control; even if SQL Server reports page 1's next page is 2, whose next page is 3, at the OS level, perhaps they're on three completely different areas of the disk. Perhaps even the OS level is contiguous, but the SAN (Meta)LUN actually has each on noncontiguous areas of the platters.

In either case, the head has to move more than it should to read three pages.