User blog comment:SlashZero/Spring 2016 E-7 Hard Pre-final & Boss Farming Composition/@comment-26273980-20160528091529/@comment-861160-20160528094627

Plane losses are always rounded down to the nearest interger. Because fighters can only be lost in proportion less than 1 to the current count, 1 slots of fighters cannot be shot down by enemies. You can exploit this for a variety of purposes, but most commonly this is the reason why people advise putting fighters in small slots because fighters are in almost 0 danger of losing ranks, and bombers which are in danger go in the larger slots.