The spelling dictionary is not meant to be a complete list of words. Some
words are deliberately omitted because they are more likely to be a mistake
(eg 'wether' is a real word, but it's more likely to be a typo). And rare
words are omitted for the obvious reason that the list needs to be kept to a
managable size. If you added the entire contents of the OED, a spell check
would take hours to run.
But having just checked it, 'soffit' is in my version of Word 2003. Perhaps
you mis-spelled it?