What are High Fly Writes?

What are High Fly Writes that are tracked by SMART on a hard drives? Does a creeping number indicate issue or upcomming issues? And, why is it not tracked on all hard drives? My Western Digital does not have this but my seagates do.

http://en.wikipedia.org/wiki/S.M.A.R.T.#Known_ATA_S.M.A.R.T._attributes says:

HDD producers implement a Fly Height Monitor that attempts to provide additional protections for write operations by detecting when a recording head is flying outside its normal operating range. If an unsafe fly height condition is encountered, the write process is stopped, and the information is rewritten or reallocated to a safe region of the hard drive. This attribute indicates the count of these errors detected over the lifetime of the drive.

This feature is implemented in most modern Seagate drives and some of Western Digital’s drives, beginning with the WD Enterprise WDE18300 and WDE9180 Ultra2 SCSI hard drives, and will be included on all future WD Enterprise products.

http://kb.acronis.com/content/9124 says:

This parameter is considered informational by most hardware vendors. Although degradation of this parameter can be an indicator of drive aging and/or potential electromechanical problems, it does not directly indicate imminent drive failure. Regular backup is recommended. Pay closer attention to other parameters and overall drive health.