That's because a lot of pins are configurable (there is a muxer on them), so you could have a maximum of 40 GPIOs but if you want say, an SPI port, then you get

That's because a lot of pins are configurable (there is a muxer on them), so you could have a maximum of 40 GPIOs but if you want say, an SPI port, then you get less GPIOs.