Why is it that it is easier to lift more weight when doing a single-armed lift, rather than a regular one? E.g. You do a dumbbell press each arm at a time, or rows each arm at a time.

Why do I ask? I was doing a workout today and I was on my third set of seated rows and decided to pick up one of the handles (as I use two free handles, one for each hand) and do a set for my left side, drop that handle/pick up the other, and do another set for my right side. I didn't lower the weight at all (120lbs) and I was able to get the same amount of reps as I would for a normal set, yet I was pulling the weight farther back than usual.

You could say that it was simply a trick of the amount of pressure on the cables, but I'm certain that I could do more weight on a single-armed dumbbell press as opposed to a normal one.

Why is that? Does it have to do with core? Is the other side of the body helping?

I dont know about that bro

obviously you can lift more each arm than both together, but only slightly. its not like you can deadlift more by not using one of your arms??

its just because your body has a little more resources to use with one hand lifts, and theres a few extra muscles helping, or your body angling whatever.
2's still better than one

Unilateral lifts engage the core in different ways, and also, quite frankly, leave greater room for 'cheating' regarding using rotational force.

The other thing that can make single limb work seem 'easier' is that your CNS has less messages to send, and recieve, so your coordination in the lift can be better.

To be honest though, if you fix your posture by maintaining a strong core, and fight against sloppy form/coping mechanisms, then single limb lifts usually dont end up any easier than double limb.

Gotcha, thanks for the replies