The pressure isn't the issue here. There is pressure in the intake at all times.
Air distribution is something totally different.
When you have air moving at the velocities involved in an engine, there can be different volumes in different runners.
If the intake is inefficient, it will be more difficult for the air to reach some runners than it will be to reach others. That will lead to some runners receiving more air than others.
In turbo applications, it means that there will be pressure differences runner to runner. The individual injectors don't know the difference. They shoot the same amount of fuel. That means some runners will be properly fueled while others will be running rich.
It's not that bad, but it can lead to a loss of power.
It's highly unlikely that there will be too much air in one cylinder. The runner pressure will not go above plenum pressure until after the valve is closed. Then it will equalize. Therefore, going lean will not be a problem unless you're not shooting enough fuel anyway.