It's percentages of what is lost
5 of 276 = roughly 2%
591 of 600 = roughly 98%
So 200k warlocks + 12k archers (roughly)
= 212k total
200k of that total = offense or roughly 93-94%
5% = archers or defense
So 96% = defense lost, 576 of 600
5% of 276 = 13.75
So with absolute base line stats it should of been ROUGHLY 576:14, keep in mind I can't say anything for certain without knowledge of bonuses/boosts etc...
However if we add +20% offense from the item boost and +40% from lost arts to the 720 it is 1209 each warlock, so you would lose without doing the math, I would guess another 20ish archers and the offense would lose roughly 6-7 warlocks which more or less lines up with what you were asking and an estimate of 590:6.
Just keep in mind that I can't see relics/crystals/boosts/paragon/lost arts etc, so I can only give rough estimations of the battle