That's the answer.
Kinematic viscosity is measured in centi-Stokes. (cSt) 1 mm squared per second is the same measurement.
If you look at the chart I cut and pasted above, you'll see just one 30 the viscosity grade column. That means straight 30 and all multigrade 30s like your 10w-30 must meet the viscosity shown to the right. So a 10W-30 should be between 9.3 and 12.5 cStokes at 100*C (212*F).
Oil viscosity decreases greatly as the oil gets hotter. So the oil grades, or weights as many people call them, are a simpler way to classify them.
