In 1755 Samuel Johnson produced a dictionary with suggested ways to spell English words. Until that time English spelling was inconsistent and haphazard. Since that time our spelling has closely followed that suggested by Johnson.
These days, spelling rules are determined by a thing called "common usage". When a new dictionary is produced, the authors will consider the ways that words are spelt by the majority of people, and write their dictionary accordingly. Contrary to some peoples' beliefs, there is no learned person sitting up in Oxford or Cambridge with a big cane in his (or her) hand laying down the rules of spelling.
Many changes to "common usage" have occurred in recent years. Even the Concise Oxford Dictionary - one of the most-used of English dictionaries, now allows alternative spellings such as:
judgment or judgement
rime or rhyme
ax or axe
tire or tyre
pajamas or pyjamas
In the 1780's in the U.S.A. Noah Webster produced a dictionary with some suggested spelling changes which have been largely accepted in America. For example, some words which previously ended in -our are now spelt without the 'u' - including: color, favor, honor, humor, neighbor and vigor.
Also, some words which used to be spelt with a double 'l' near the end are now spelt with just one - for example: leveled, traveled, traveling, jewelry, woolen, marvelous.
Some words which ended in '-re' are now written '-er' - for example: center, theater, meter, fiber.
And some words which ended in -ce and now acceptable with -se - including: defense, offense, and even practise (the noun).
And other words now have variations in spelling which have become accepted around the world, including:
catalog instead of catalogue
mold instead of mould
jail instead of gaol
plow instead of plough