In a pressurized portafilter there's usually a plastic or rubber device located below the filter basket that forces the coffee to pass through small openings, thus generating pressure. That way it creates foamed coffee (bubbles made of air) that has nothing to do with a crema, it just gives the illusion of it even with stale preground supermarket coffee.
Crema is an emulsion of water and aromatic oils that forms tiny bubbles around carbon dioxide released from the coffee beans during extraction when water is pressed through finely ground fresh coffee at around 9 bar and the right temperature (around 90°C/194°F). Since extraction has already taken place when pressure develops in a pressurized portafilter, no crema can arise. It may look like it, but it certainly doesn't taste like it. It's a fake. You can taste the difference.
A non-presurized portafilter is less forgiving do, because your grind and tamp have to be correct.