I don't think SC2 has anything to do with this.
It does because they have the same Gryphon Rider model there as well. Would be interesting to know if this error is present there as well, or if they fixed it.
Unlike WC3 where textures were generally bundled with their models making cross-linking textures sometimes painful, SC2 throws all textures into a single folder.
Ok so far...
The actual Gryphon Rider texture is 256*256 (GryphonRider.blp).
The RoC Gryphon Rider texture was overwritten by TFT. Both RoC and TFT versions of the texture are pixel identical. This means that technically the texture is included twice in the WC3 install (a waste of space).
The Gryphon Rider Missile uses a 512*512 version of the Gryphon Rider texture (Griffon.blp).
Analysis of the 512*512 Gryphon Rider texture with respect to the 256*256 version shows nothing more than down sampling and compression artefacts meaning that the 512*512 is likely the original painted texture.
SC2 includes both 256*256 Gryphon Rider (war3_gryphonrider.dds) texture and 512*512 Gryphon Rider Missile (war3_griffon.dds) texture complete with correct normal maps.
In SC2 the Gryphon Rider model and portrait use the 256*256 texture. One should be able to use texture overrides to replace this with the 512*512 hopefully.
In SC2 the Gryphon Rider Weapon model uses the 512*512 texture.
Conclusion? Blizzard is quite lazy I guess...