I'm not entirely sure where you get your math from..
Boosting items in Pokemon boost the final stat. Tyranitar has 403 attack at lvl 100. With a Choice Band it becomes 604. If TTar's BASE Attack was multiplied it would end up with 629 Attack, which is wrong. Same with Choice Scarf and Evolite. It just 1.5x the stats which is simple math to find the resulting base stat (even though base stats never change)
stat modification goes roughly in this order. Base stats --> IVs/EVs/level --> Items/boosts/abilities