After running some drives for a whole day, I noticed that power-on hours (SMART attribute 09h in some drives), has only increased 2 hours, which is really strange, so, I checked a manual for an Intel SSD, to find what does the SATA SMART attributes means exactly, and found this:
09h Power-On Hours Count
The raw value reports two values: the first 4 bytes report the cumulative number of power-on hours over the life of the device, the remaining bytes report the number of milliseconds since the last hour increment.
The On/Off status of the Device Initiated Power Management (DIPM) feature will affect the number of hours reported. If DIPM is turned On, the recorded value for power-on hours does not include the time that the device is in a “slumber” state. If DIPM is turned Off, the recorded value for power-on hours should match the clock time, as all three device states are counted: active, idle and slumber.
This is unlike the old drivers, where the power on hours count represents the total number the drive has not been turned off, currently, as-if DIPM is turned Off.
DIPM is a power saving feature that is enabled by default on multiple operating systems, including Windows 8.1.
This probably applies to many of the new hard disk drives and sold state drives, specially drives with sandforce controller, you should check your SSD or HDD model manual for accurate data.