For a good chunk of spring CPK isnt fully leafed out yet, so the temp disparity doesn't show as much yet compared to nearby urban stations. Also in spring the Park is a little more protected from the frequent cool E/NE flow, keeping LGA cooler on some days. As that influence gets weaker later on, back door fronts less common, and the vegetation gets lush the Park's cool bias starts on cue.
I wish I could find the article, but the NWS has already acknowledged this phenomenon is legitimate 10 yrs ago or so. They basically just accept it as is for a few reasons...
1. Technically KNYC is accurate for what it is, a dense semi forested park, and its readings accuratly reflect that. (Kinda putting the onus on the consumer to recognize a park for what it is vs an urbanized street and analyze accordingly.)
2. Its historical significance. Its one of the longest running climate stations, and in America's biggest city. They dont want to mess with it. So its just grandfathered in to the climate data despite falling short of their own offical standards.
3. There are plenty of offical weather stations around it to reflect the local urban climate. (Kind of putting it at the medias feet to use all available data to portray an accurate picture to the public.) In fact the closest offical reporting station for many/most NYC residents isnt KNYC, but LGA or JFK...all of Queens, the BX and portions of BK would fall into that.
The ASOS is not that far from Museum of Natural History. Its fenced in, surrounded by trees, last I saw some now hang over the top as well. In wetter patterns weeds and shrubs will be overgrown near-by too. The trees do help shield it from the public which the NWS probably likes.