Parity between handheld and docked modes makes sense for why CPU clocks are close, but there is no reason for the GPU. Games scale fine by resolution. There isn't much of a difference between scaling a game 2 times the resolution and 2.5 times or 3 times. That's essentially what maxing the GPU in docked mode would do.
The reason why the GPU clocks aren't maxed likely is mostly due to power consumption and thermals. Nintendo likes to keep their thermals in the 60's-70's Celsius (as opposed to other systems that let them go up to 80's-90's and throttle) and they are very much about maintaining optimal power consumption. Increasing the clock rate to its max, probably super-linearly increases power consumption. And yes Nintendo probably could safely have the SOC run 80c+, but they aren't one to do that.
