What is the difference between 30 and 40 volume cream developer?

The difference is the strength of the developers.

A 30% developer will help you achieve a minimal lift and a 40% will make the color brighter/louder which will result in a maximum lift.

In other Words a 30 would lift your hair 3 times and a 40 will lift it 4X times the color. When coloring your hair like this it goes through stages of lightening that's why stylist put it on & check back to see if the color has made it to the desired color that you want or they want to achieve for you.