I mean, they're not even close. How did that happen?
As best I can tell, the problem is that the spellings and pronunciations come from separate dialects. (The same sort of problem is present in words like
bury and
business, where the pronunciation of the
u is anomalous.) Unfortunately the
OED is not very helpful when it comes to pronunciations across different dialects across time.
I
think the spellings both come from the south of England, and they presumably had the same vowel sound. At the end of a word following a low vowel, the
gh combination (which was pronounced like a German
ch) typically became /f/. So
laugh has a non-silent
gh, and when it's expanded to
laughter, the pronunciation stays the same. In
daughter, however, the
gh comes in the middle of the word and thus was lost.
And as for the first vowel of
laughter, our pronunciation of that word comes from Anglia, where the vowel was fronted. The spelling was probably already established by the time that pronunciation became common, and you know how impossible it is to get any spelling reform done in this language.
