That's how SMART attributes work. 100 means AOK and 0 means failed. The normalized number is reported by the drive, calculated by a formula based off the raw values and MTBF data determined by the manufacturer.
ID # 179 Used Reserved Block Count (total)
This attribute represents the number of reserved blocks that have been used as a result of a read, program or erase failure. This value is related to attribute 5 (Reallocated Sector Count) and will vary based on SSD density.
.. so at least for samsung there's use of exact numbers.
In Smartmontools I found the code for this variable:
http://smartmontools.sourceforge.net/doxygen/atacmds_8cpp_so...
It's code 179 (0xB3).
From Samsung's website:
http://www.samsung.com/global/business/semiconductor/minisit...
.. so at least for samsung there's use of exact numbers.From Intel's website:
http://download.intel.com/newsroom/kits/ssd/pdfs/intel_ssd_5...
(Ctrl-F for "Available Reserved Space")
.. they use a normalized value (100).
So it can be either percentage or absolute value, depending on manufacturer.