Placement
Where your ad appeared — top of search, rest of search, or product pages.
The same keyword performs very differently by placement. Top of search usually converts best and costs most; product pages usually convert worst.
Amazon lets you apply a percentage modifier per placement, so your effective bid is base bid × (1 + modifier). At a $1.00 base, a 15% top-of-search modifier means $1.15 there and $1.00 elsewhere.
Bids and placement modifiers are one system. Cutting bids without touching modifiers is the most common cause of "I lowered bids and my ACOS went up" — you lost top-of-search, where the conversions were.