CT state tax return looks at all of your income from all sources and generates a tax on all this income. When CT is determining the actual CT state taxes owed, CT multiples the total CT taxes on all income by the percentage of CT source income over all source income.
So if your income from all sources is $10,000 but your CT source income is $8,000, CT will calculate the CT state tax on $10,000 then multiply the total state taxes generated by 80% ($8,000/$10,000). So even though you all your income is being reported on your CT state tax return, you are only being taxed on the CT source portion of your total income.
Please see the screen shot below: