It’s basically up to whoever is doing the transliteration. Arabic letters don’t have exactly corresponding English counterparts, and as far as I know there is no Official Guide to Arabic-English Transliteration (follow it or be visited by serious men in dark suits).
I was writing a paper on Genghis Khan and found that one book credited an event called The Long Ride to one Chepe, whereas another credited it to one Jebe. I was pretty confused before I realised they were the same guy, transliterated differently.