Companies install solar systems to earn money, and naturally the bigger the solar installation the better, within cost and other restrictions such as output thresholds. There is therefore a tendency to fill a given area with solar panels up to a reasonable amount, regardless of the real world effectiveness of each panel.
Online quotes are perhaps the worst for this and there is little combined planning between customer and installation company, except for secondary issues such as which battery and what the technical installation looks like.
So what is the issue?
Shadow, angle and seasons. The combined effect of these is very difficult to model in all but simple situations such as a perfect, shadow-free south facing roof.
Shadows cast by surrounding buildings, trees, and other structures can significantly reduce the amount of sunlight that reaches the panels, affecting their efficiency and energy output.
Models of the impact of the angle of the solar panels to the sun are also unreliable; for example many don’t take into account the combined effect of roof slop and direction.
The result is that the true output of a solar installation in a complex placement can be dramatically less than forecast, thereby reducing the cost effectiveness of the system.