Some of my dispersion colors are so thin it's like water and others are thicker. Why is this?

All dispersion colors are a different viscosity and will require different amounts of pigment. Some are more transparent and some are more solid colors.  The more transparent the color the thinner the consistency will be.  However, with our neon collection it will be thicker and require more mixing/stirring prior to use due to what pigment is needed to give that vibrant neon color you love.