There are several parameters that affect this, such as the consistency of the concrete, temperature, ventilation, etc. But the speed of pouring also has a great impact. There may also be some differences between how it is pumped into the formwork.
If you have a sluggish (sticky) concrete that may have time to stiffen (high temp), and you pour slowly, the formwork pressure can below. If, on the other hand, you pour quickly with a loose consistency, the pressure can become high.