I suspect that this may vary depending on your country of residence, industry and union, but what you're describing is at least not how it works in my country a

I suspect that this may vary depending on your country of residence, industry and union, but what you're describing is at least not how it works in my country and within the unions I've been part of. My situation is such that the union together with the employer negotiate a base agreement. It might detail things like making sure you received compensation for overtime, what the requirements for termination are, rights in regards to parental leave, minimum annual wage adjustments, etc. My country has very little legislation in regards to workers rights, because we've traditionally had very strong unions instead, so workers rights could be mandated through agreements rather than law. To give you an ideal, the standard collective bargaining agreement within my industry is a few hundred pages. If you are not part of such an agreement, it is instead on you to make sure that your employment contract covers all those aspects. The benefit for the employer is that they have to deal with less legislation, don't have to negotiate a complex contract with every employee (i.e. we already agree on the vast majority of issues, and can focus on the few things where I want individual adjustment), and finally, as long as the employer follows the agreement, the workers through their union agree not to go on strike. Now, to your question, just because you have an agreement through your union does not mean that you cannot exceed that agreement. You can certainly negotiate a higher salary, for example. It only ensures the minimum level.