Another WTF moment

[Ronald F. Guilmette]

I have two Seagate ST380011A drives, both in the same single system.

On that system, I boot to the FreeBSD 9.1-RC3 LiveCD.

The resulting dmesg messages indicate the following regarding the two drives:

ada0 at ata0 bus 0 scbus2 target 0 lun 0
ada0: <ST380011A 3.54> ATA-6 device
ada0: 100.000MB/s transfers (UDMA5, PIO 8192bytes)
ada0: 76318MB (156299375 512 byte sectors: 16H 63S/T 16383C)
ada0: Previously was known as ad0
ada1 at ata0 bus 0 scbus2 target 1 lun 0
ada1: <ST380011A 3.06> ATA-6 device
ada1: 100.000MB/s transfers (UDMA5, PIO 8192bytes)
ada1: 76319MB (156301488 512 byte sectors: 16H 63S/T 16383C)
ada1: Previously was known as ad1


So, um, WTF?  One ST380011A is 156299375 sectors big, and the other one
is 156301488 big.

How exactly does this happen?