oh gosh. Aisha, ALL YOURS singles where definatlynot fillers.
Kitto Eien ni + Konna Chikaku de... has to be up there with her greatest songs ever.
m-flo's popularity helped the Two as One sales.
and Crystal peformed Kitto Eien ni + Konna Chikaku de... only once and Anata no Soba de, well, she didn't.
whereas with 恋におちたら, she performed it dozens of times..

i think the answer is promotion tbh..