I bought 2xBlack Dog Machine 10rds magazines from Midwayusa(AR-SM-10R)
but both mags only hold 9 rounds max, not 10
Why is that?

Probably needs to be broken in, or the rounds are being stacked weird.

The bolt hold open follower is taller so one round is lost. The magazine you have was designed before the bolt hold open feature. You can buy non- bolt hold open followers on our website.
The new magazines will have all rounds and BHO feature.