ST MCU Calculating external capacitor values for oscillator on STM32F2
The datasheet says Cs which includes pin and PCB wiring capacitance can roughly be approximated as 10 pF.
Therefore when a 6pF load is needed, you need 12pF of capacitance, and subtract 10pF stray, and you are left with 2pF capacitors.
The other crystal is not even mounted, but it might have more stray capacitance of 11.7pF, so 4.3pF capacitors are enough to get 16pF in total.