^I think that Japanese blog (Natsu Style) points out really well. Since avex started doing those FC special (e.g., teamayu preorder special), the first day sales (or preorder sales on Tuesday) is always very high, then the second (real release day) is always low. Back then the first and second day indexes are pretty close to each other. I think now avex is kinda similar to JE. First day huge index, then a sharp drop on the second day.
I think this doesn't necessarily apply to other companies tho. At least EMI doesn't have fan club preorder special for hikki, nor Sony handed out special gifts if you preorder Mika's VOICE
.