A explicação mais provável é que a Seagate usa atributos SMART de forma proprietária.
A Seagate tem isto para dizer :
The SMART values that might be read out by third-party SMART software are not based on how the values may be used within the Seagate hard drives. Seagate does not provide support for software programs that claim to read individual SMART attributes and thresholds...
...some third-party SMART software programs display a list of attributes that seem to announce or foreshadow a SATA hard drive failure... please remember that these third-party programs do not have proprietary access to Seagate hard disk information, and therefore often provide inconsistent and inaccurate results. SeaTools is more consistent and more accurate and is the standard Seagate uses to determine hard drive failure.
Parece que a Seagate está muito feliz em usar os atributos SMART da maneira que quiser.