Jolly 252 Atk ScarfTar = 367 AtkXxBraedonxX, he doesn't have one less attack than Tyranitar. He has 17, which IS a big difference. He has about 365 max IIRC, and ttar has 403. Also, TTar has STAB Stone Edge and a lot more resistances, as well as Sand Stream
Adamant 252 Atk ScarfWaru = 366 Atk
If you notice, it was running Adamant instead of Jolly like what ScarfTar generally does. As with Adamant ScarfTar doesn't outspeed Timid Gengar, Timid Starmie, and Modest Alakazam (maybe important once it gets Magic Guard).
Jolly 252 Spe ScarfTar = 364 Spe
Adamant 252 Spe ScarfWaru = 424 Spe
While Adamant ScarfWaru outspeeds all but +speed Agirudaa, and also +1 90s without a speed boosting nature (for revenge kills/countering), plus all Scarf Heatrans. Jolly ScarfWaru speed ties with +1 113s without a boosting nature (Jalorda, thou it wouldn't switch into it anyway)
The 342/256/236 defense of ScarfTar compared to 322/176/176 ScarfWaru make it basically a choice between Speed and Bulk as much as between secondary STABs.