This is probably really easy. Somebody tell me where I'm wrong. (if)

Trade offered to me: COH AP 15%ED 971 def

According to arreatsummit AP has max def 524, which is enacted plus one due to base stat 15%ED

525 * 15 + 525 = 603.75

Runeword COH has +70%ED

603.75 * .7 + 603.75 = 1026.375

Where am I confused?

Both ED are calculated together, not one after another.

525 * 1.85 = 971.25

There it is. TY for the reply and info!