For Several years the state has decided to correct my federal return and charge me a lot of money In state taxes on a small amount of self employment.How can they do this

I make roughly 16k in self employment and every year state has come back saying I am not owed a refund but instead I owe them 400-600$. They claim to adjust my federal tax every year and my agi. How is this? My returns are fairly simple and not real complex with a lot of deductions or anything. I have 2 dependents one is a child under 16 one a disabled child over 16