High Reallocated_Event_Count with Reallocated_Sector_Ct == 0?

The most likely explanation is that Seagate uses SMART attributes in a proprietary way.

Seagate has this to say:

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.

Seems like Seagate is quite happy to use the SMART attributes any way it wants to.