Put one value in PORTA, and another value appears (PIC24 from Microchip)
You didn't supply a link to the datasheet, so I didn't look it up.  My guess is that not all 16 bits of port A are implemented.

Port A pins also are sometimes used for the oscillator pins, the MCLR input, etc.  Those aren't available as port A digital pins when those other roles are used.

You really need to read the datasheet.