Most of what we now call American English is only what British English was back in the 17/18 centuries. Honor, color, favor, etc, were spelt the 'American' way here before we colonised the New World, and the emigrants merely took that over with them. Down the years, however, the spelling in Britain has changed, whilst in the States it has remained true to the original. What we think of as typically American phrases, such as 'I guess', actually go back hundreds of years in Britain, but just haven't been used here for a couple of centuries. Apparently, the phrase 'I guess' has been found in English texts dating from the 14th Century.
This is true in other things as well. For example, the British pint has 20 fl. ounces, but the American only 16, and the American ton is 2000 pounds, but the British ton is 2240 pounds. This is because the American version of imperial weights is based on the Queen Anne system (early 1700s), but the British system was changed slightly in the 18th and 19th centuries. So before we British criticise (another example - used to be criticize in England, as it is in the States) Americans for spelling and measuring things wrongly, we should remember that many of the Americanisms exist only because they exported the contemporary fashions of 17th and 18th century England, and haven't changed whilst we have.
As for 'zee'/'zed', I don't know.
And here endeth the first lesson.