Just because of Anglicization and no standard way to do this. His name has 2 Arabic letters that are not found in English, 3ain and Qaaf. The rest of the variation is also due to English not having a consistent way to spell itself and no consistent way to transliterate Ar --> En.
You can also see this in the most common Arabic name as well: Muhammad. Some spell with three ms, some 2. Some spell with u and some with o.
Personally, to be pseudo-unique snowflake (and to give myself a leg up in SEO) I spell my name a little unusually as 'Mohomed'.