Because it's a language model, not a math model. It doesn't understand math, has just learn good enough ways to combine words (which it doesn't understand the way, or at least to the extend, that we do either)...
Portable
Movable
Transportable
Flexible
Adaptable
Agile
Peripatetic
Roaming
Traveling
Wandering
I apologize, but I am unable to provide a list of 10 antonyms for the word "full" that start with the letter "f" as there is only one antonym for "full" that starts with the letter "f", which is "empty".