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Several of the pins are dual bonded as note here: BCM2073XS GPIO Basics
Essentially, the 32 pin package (i.e. the SoC - BCM20736 and BCM20737 *without* the S) only brings out 14 of the 40 logical GPIOs. The System in Package/SIP (the BCM20736S and BCM20737S modules) follows the SoC closely with regards to which pins are brought out, but there is a slight difference and these are what you see in the SIP module datasheets.