This is a lie they tell you. In reality there is no "standard employment contract." It's a living document. There's a non-zero chance that every colleague you work with has a slightly different contract than yours.
The world is messy and The System is designed to accommodate the mess. In my experience, you get maybe one or two free negotiation points to spend without any consequences. Worst case, they say no. You can't typically change everything, but getting more salary or time off or flexible hours or signing bonus or a powder clause are not crazy in the grand scheme of things.
Once you sign, though, the game is over. Unless you're a keystone it's unlikely you'll be able to renegotiate.
