How is root node value chosen in regression decision tree?

I understand the criteria for node splitting and how the root node variable is chosen but I do not understand how the actual value for the inequality at the root node is chosen. Is it just local optimization of the numbers? For example, I have a variety of whole number values ranging from 3 to 25 and the root node is chosing 9.5. This is not the median or mean, so why is this number chosen? Is it because the decision tree analyzed all potential values to see what had the lowest MSE to start with? If so, why did it chose a decimal number when all my data points are whole numbers?

Thank you for your help!