i'm assuming with the bar it helps you stabilize a bit whereas the dumbell it's all freemotion so you have to engage your core and balance which takes away from being able to give off full strenght?
Pretty much. You end up using some of your strength to stabilize the weight more with dumbells. Making it harder to lift the same amount of weight.
Same goes for the machines. It's much easier to do leg presses on a sled than doing squats with a barbell. If I use the sled I pull nearly every 45 plate in the gym to use (540lbs) leaving noone else any weights to use.
Squating it's more around 275.