That form does not calculate social security tax. It calculates an estimated self employment tax. None of the numbers on that page are the amount of tax owed in social security. Lines 5-9 are calculating how much additional tax you can deduct because you are paying social security tax on the first 184.5K of your income. When your income exceeds 184.5k, you are not paying additional social security tax, so you are not entitled to a greater deduction against your self-employment tax.