The ones in the top pic are heavier.. the ones in my gym are 5lbs heavier than the normal dumbbells... yes i told the manager that the weights seem off, and we weighed some... the ones over 60lbs are usually 5lbs heavier... im not saying this as a rule of thumb, but at least in my gym thats how they are... maybe a bad batch?
Generally, because the plates are cast to be as cheap as possible, the tolerances are very loose, so a 5lb plate might weigh 4.85 lbs, while another might weigh 5.10 lbs, etc...... Generally when you cast metal, a small item will be closer to weight than a large one(assuming the molds are in good repair), so most companies just shrug it off. On larger items the weight might swing 3-5 lbs on a 45lb plate or even more on a 100lb'er.
That's why you see such a big jump in price on better and competition grade plates, because they are milled down to close to exact weight. York and Ivanko have a tolerance of 2% or so on their gym plates, and few grams on their competition plates that you see at big meets.
