Losing ~2% of your entire army is pretty significant. By some estimates it takes a loss of 10% to be in dire need of relief, and an army that's lost 30% simply cannot fight effectively. Not everyone in an army is even necessarily a combat soldier.

And of course at any given time there's only so many troops on the "front line". There's only so many troops you can have actively engaged in combat that you can effectively move, retreat, supply and generally just control.

Finally, you always want to scrutinize statistics because you're always told them, but not their methodology. To be precise- unless someone was shot directly in the head, heart, throat, or lungs, they probably would survive at least for a day. Weapons of the era were not precise. By one method someone who didn't die in the immediacy of the battle might be tallied as a wounded even if they still died of said wounds even days later.

If you want a classic example of conflicting statistical gathering, compare German and Soviet records for WW2. A soviet T-34 that blew out it's engine when it got stuck in the mud, then was abandoned and reclaimed, then shot and abandoned again to be recovered and then finally lost when it got hit again, detonating it's ammo could be marked as four losses. A German Panzer 4 that blew out it's engine when it got stuck in the mud, abandoned twice, and then a third time might still not be marked as a loss until all operations in the area ceased.

First of all, please make the distinction between "casualties" and "killed". As the other poster mentioned, KIA might not be the best way of measuring "casualties" because most of them would be wounded.

Second of all, 7 days for the Union isn't really the best example of a bloody battle. If you look at Gettysburg for instance, around 25% of the union and 1/3 of the Confederate army were casualties. At Antietam around 1/4 of the Army of Norther Virginia were casualties.