The claim is that your wages have been negotiated with the expectation that some portion of them will be paid to the government, hence that portion does not belong to you by contractual agreement (it doesn't belong to you ethically, nor legally either, but those are other arguments addressed separately).
The fact that you expect or assume a portion of my wage will (or should) be payed to the government is question-begging. Your expectations, assumptions, and other mental furniture do not factor into any contract unless it is written or stated explicitly and agreed upon. If there are such explicit expectations then maybe you can furnish an example.
While it is true that employment contracts often contain a note that a salary is subject to deductions and taxes, this is to inform the employee of what will happen to a portion of his salary, not to declare any right or property of the government. Employers deduct from an employee’s compensation because they face fine and punishment if they do not, not because that compensation is in fact the government’s property. And the question as to whose compensation they are deducting from is a silly one.
It doesn’t matter what “everyone involved knows you will take home in compensation for your labour” unless it is explicitly stated in the contract or agreement. Again, your assumptions, expectations, and what you think you know is merely question begging. The “gross wage” is what is agreed upon as the compensation. The “net wage” is what’s left over after the government has its way with it. If you and your employer negotiate $100,000 a year, that is the gross salary, the total from which taxes are deducted. It doesn’t mean you tacitly agreed to a less amount.
At any rate, payroll deductions and taxes are calculated after the wage is determined, as the determined wage is required to calculate the cost of deductions in the first place. These deductions are determined by the government, are enforced by coercion, in most cases extorted by fine or other penalties.
So if all parties agree to the wage, the wage is then payed for services rendered, the money will exchange hands from employer to employee, and what was once the employer’s property is now the employee’s property. Finally, the government takes the employee’s property.