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I guess I can answer my question meanwhile.
Fdisk under RHEL 6 or Oracle Linux 6 has a DOS compatibility mode. It can be turned off (and recommended by the man page) by using the -c parameter.
fdisk -clu /dev/sda
Disk /dev/sda: 21.5 GB, 21474836480 bytes
255 heads, 63 sectors/track, 2610 cylinders, total 41943040 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000a2ccf
Device Boot Start End Blocks Id System
/dev/sda1 * 2048 1026047 512000 83 Linux
/dev/sda2 1026048 41943039 20458496 8e Linux LVM
The question might be why such a setting was added or why it not default, but I guess it explains what I experienced.
It's been a while since I looked into the issue, but as far as I remember it is just that particular version of fdisk in RHEL 6 which does not have DOS compatibility mode disabled by default. Using the -c parameter should disable this warning.
DOS compatibility mode is obsolete. There is no need for CHS addressing, which was used for very old disks using a floppy disk schema. All modern hard drives use LBA, so the values fdisk uses are going to be translated by the disk anyway to match the actual disk alignment and geometry.