How is climb rate on the stat card calculated?

It feels like a lot of the stats on the plane card is wrong, for example the Ki-44 is rated at 18.8 m/s while the P-51 is at 22 m/s. But from my experience the Ki climbs so much faster, at a much steeper angle, reaching the battlefield a lot higher than the P-51 would. I know this is expected as the Ki excels in acceleration while the Mustang is a heavier and faster aircraft, but I wanna know where the number on the stat card comes and if it's straight up wrong. I'm talking about the P-51K btw. If it really is wrong, where can I find reliable information on how well an aircraft climbs?