Yes, there are "invisible letters" in English words. There are also silent letters in English words, letters that are clearly there but are not pronounced. And other letters that don't really make a sound but change the way other letters are pronounced. I am sorry to say that English spelling makes no sense at all. The answers to all questions about English spelling and pronunciation are "it just is," or "history." (Yes, in many cases the spelling reflect something about how the word was pronounced a century ago). Native English speakers in the U.S. learn by a combination of sight-reading and "phonics." The spelling of a word is, at best, a _memory aid_ for the way it is pronounced. There are dozens of examples of craziness in English spelling. No, hundreds. Thousands. I sincerely apologize to you, on behalf of the English language. :)

English originates from Germanic languages and after the Norman invasion of England in 1066 was heavily influenced by Norman French. There was until the invention of the printing press little agreement on how words should be spelt. All this combines to create inconsistencies in spelling. You just simply have to learn to live with English spelling and accept it is not a phonetic language and that it is often not written as it is spelt.

Languages are Oral before they are put into written form. 1 was pronounced won, before they wrote it as one. So the correct answer is "Why did they spell it one?"