In word like ‘solid,’ the ‘o’ is in tense context, and the ‘i’ is lax. This implies it should be pronounced so-lid, which it obviously is not. In order to maintain some sense of integrity, and extra value for readers, we would like to fix that. In many cases in English words, the tenseness of a vowel is not that clear. For example, in ‘antediluvian,’ is the ‘e’ pronounced lax or tense? Arguments could be made either way, and not very productively. The lexical context is tense, and that seems good enough. The stress in this word is on the ‘u,’ and it’s very clear that it’s pronounced tense, which matches the lexical context. So all is well, and we can spell the word ‘antediluveyan.’
Other words have exceptions in both directions. In the word ‘wholly,’ the lexical context of the stressed vowel is lax, but the pronunciation is tense, so we will spell it as ‘wholy.’ The word ‘ghost’ is similar, but there are 2 consonant sounds, so we can’t make a lax context there. Instead, we have to use the 2-letter form, and write ‘goast.’
In the word ‘solid,’ it’s the other way round, so we spell it ‘sollid.’