If you try to create a Skeleton with a Bow that should make more damage (without enchantment), the Bow doesn't use the generic.atta­ckDamage Attribute. I tested it with exactly the same spawn only a differnet weapon (diamond_sword). The Attribute works fine with the diamond_sword.

Summon Skeleton with Bow:
summon Skeleton ~0 ~1 ~0 {Equipment:[{id:"minecraft:bow",tag:{AttributeModifiers:[

{AttributeName:"generic.attackDamage",Name:"generic.attackDamage",Amount:20,Operation:0,UUIDLeast:894654,UUIDMost:2872}

]}},{},{},{},{id:leather_helmet,tag:{display:}}]}

Summon Skeleton with Diamond-Sword:
summon Skeleton ~0 ~1 ~0 {Equipment:[{id:"minecraft:diamond_sword",tag:{AttributeModifiers:[

{AttributeName:"generic.attackDamage",Name:"generic.attackDamage",Amount:20,Operation:0,UUIDLeast:894654,UUIDMost:2872}

]}},{},{},{},{id:leather_helmet,tag:{display:}}]}

I don't know enought about the parameters so maybe the damage comes from the projectiles? I tested it with different Amounts but it doesn't change anything.