Some whistles aren't tuned to equal temperment. Unfortunately, that's what your electronic thingie reads, so the whistle appears to be out of tune with itself. It means it could be a bit iffy playing with fixed pitch instruments, but most people won't really hear the difference.

It's also possible to change the pitch a bit by blowing harder or softer, so it could be 'operator error'.

Some are just rotten whistles, but Feadog and Walton's are supposed to be good ones.

I've tried the hot water dipping thing, and not melted the mouthpiece on average of 1 time out of 4. It may be a personal problem. Go to the Chiff & Fipple site Blackcatter linked to - it has a ton of information, and it's just plain fun to read.