I don't really think its about guessing. People with perfect pitch don't guess. Its like naming a colour or naming the height of something. Its like colours have shades and you can recognise them red or blue or green. Sounds have pitches and tones and perfect pitch people can name them just like colours.
I could do that last time when I was learning piano.
But erm... I think when I learnt a Chinese instrument, I got used to the 5 tones (Western music has 12 notes in a scale but Chinese music only has 5). And when I did, I couldn't detect notes other than these five accurately. And if anything out of these five comes about, it would confuse me and then I would not be able to name the notes accurately after that...
I think its just like if you lived in a world where there are only Red Blue Yellow. Then you are told to name orange, you'll be thrown off balance and then erm... you forgot what colour is red and yellow (Sounds impossible but I think that's my case XD)