So I noticed the tank Rhyperior has 28 Atk investment, which it says is supposed to be for offensive Drapion variants. However, no Attack investment is actually needed at all:
0+ Atk Rhyperior Earthquake vs. 0 HP / 4 Def Drapion: 290-344 (103.2 - 122.4%) -- guaranteed OHKO
28+ Atk Rhyperior Earthquake vs. 76 HP / 0 Def Drapion: 300-354 (100 - 118%) -- guaranteed OHKO
24+ Atk Rhyperior Earthquake vs. 76 HP / 0 Def Drapion: 300-354 (100 - 118%) -- guaranteed OHKO
28 Atk investment only seems to target some weird 76 HP Drapion variant, but 24 Atk investment takes that down all the same. So I thought it'd be more for Tyrantrum, in which case...
28+ Atk Rhyperior Earthquake vs. 0 HP / 4 Def Tyrantrum: 278-330 (91.1 - 108.1%) -- 50% chance to OHKO
0+ Atk Rhyperior Earthquake vs. 0 HP / 4 Def Tyrantrum: 270-320 (88.5 - 104.9%) -- guaranteed OHKO after Stealth Rock
28 Atk investment doesn't come close to securing a flat OHKO, and after SR, the Atk investment is no longer required either. So yeah, I have no clue what 28 Atk EVs does specifically.