It has more available GPIO pins according to this site :
RP2040-PICO30 is RP2040-PICO with 30 GPIO EXPOSED
www.olimex.com
According to text :
"
RP2040-PICO30 is re-design of the popular Raspberry Pi Pico (RP2040) but exposes all 30 GPIOs
RP2040-PICO30 comes with 2MB flash memory as original RP2040-PICO, but there is also variant RP2040-PICO30-16 with the maximum possible flash memory for RP2040 - 16MB.
RP2040-PICO30U has also mounted UEXT connector and LED. The LED is not connected to any port by default, but can be connected to GPIO25 as in the original RP2040-PICO via solder jumper. There are two 2.2K pullup resistors options to the SCL and SDL ports (GPIO16 and GPIO17).
"
The pdf manual explains it all, some pins are GND with the original RP2040 and have become GPIO pins with the RP2040-PICO30.