Small weight differences.

I got myself 6 discs and all weighed individually. This led to slight variations (about 0.1 to 0.2 kg over / under 10 kg)
Well, is bearable for me.
The discs are not quite uniform. The surface is slightly wavy, but by the closure of my weight does not bother.

For Use may not say much, just hanging on the pole :-) and fulfill your purpose, namely to be hard ....