I'm new to cakes. I've noticed my cakes where I beat the whites by hand with a whisk tend to taste moister than when I do so with my hand-held mixer. I wonder if this is general knowledge, or just my impression. I don't seem to get as stiff a peak with my hand whisk -- it's always just firm. But the final product has a nicer texture. Also, I tend to do small cakes that use 3-4 eggs, so that might be one reason.
Could you be over-beating the egg whites with the mixer?
Oh my! Just trying to remember when I hand beat egg whites the last time... Must be almost 30 years ago in grandma's kitchen. It took forever, was always my duty and I hated it, lol. Using the same recipes today (and an electric mixer), I honestly won't notice a difference (despite not suffering muscle pain in my arms, hehe!). As said above the reason you might notice a difference might be that you "feel" the ingredients more when mixing them by hand and therefore you might have better control in regards of over or under mixing.