Installed capacity is a misleading number. If you assessed the trucking industry by simply sum-ing the rated capacity of all the hardware you'd be rightfully laughed and and called a liar on the basis of all the times the trucks are empty and all the trucks that run out of volume before weight. Renewables is a similar situation.

Some panel in a solar farm in Canada is not gonna see the conditions that let it produce rated capacity nearly as often as one in Arizona. So the guy in Canada installs more capacity to get the same power. Meanwhile the guy in Arizona doesn't have enough copper leading out of his site to handle the power he could produce at peak on the best days, because he over-provisioned too, in order to be able to produce a given amount earlier/later in the day. The actual generation hardware is so cheap that this is just the sensible way to deploy renewables, but it makes for stupid misleading numbers.

Legacy power generation has much different numbers and isn't subject to the whims of the weather so installed capacity is a number that means something in that context.