Just to add onto what BoredSaint has said:
Increasing stock turnover effectively means you are converting stock to sales in a quicker timeframe. If stock levels are not replenished quickly enough, then overall stock balance will be less. As a result, lower stock levels may indicate improvement in the STO.
Theoretically, if you hold less stock on hand that is not due to an improvement in your ability to convert stock to sales, then STO won't increase. (Although this is very rare). For example, a business may order less stock due to lower consumer demand, and this doesn't actually improve the STO.
As for ROA, assuming your STO increased because you're converting stock to sales on a more regular basis, then net profit will increase in the form of sales. Since ROA = Net profit/Assets, an improvement in the STO may have a favourable effect on ROA.