Board Thread:League of Legends discussion/@comment-26107879-20180824001037/@comment-3391671-20180824063402

Bonus AS from levels are considered as "Bonus" unlike other stats where additional stats are part of the base stat (for example, Health). Thus, when you gain 100% bonus AS, you don't multiply it by the champion's current item-less AS, but by the AS they have at lvl 1.