Father of arab chemistry

It probably has something to do with the way that things were spelled back then. Phonologically, "Jabir" is close to "Geber" so the people back then who couldn't spell his name the way Islamic scientists did spelled it as that. They even had the habit of making feminine-like names into muscular.

When Jabir ibn Haiyan's name was first transliterated, the letter 'J' did not have the 'dzh' sound that it is now used for. It was, in fact, a terminal form of 'i' (you will be familiar with terminal forms of letters, since you know Arabic) and was used both for the sound 'i' and the consonantal sound 'y'. The letter 'G' at the time had two values (as it does today): the soft sound, 'dzh', as in 'general' and 'genuine' and the hard sound 'g' as in 'give' and 'get'. The only wriiten vowel in the Arabic original is the 'alif which is transcribed in Jabir's name as 'a' macron (the letter 'a' with a line over it to indicate a long 'a' sound as in the word 'mate'. In mediaeval Europe, this sound would be represented by the letter 'e'. The vowel between the 'b' and the 'r' is not written in Arabic but is a short vowel which is something like both 'i' and 'e' to an European's ear. The name 'Geber' is therefore an old version of the modern transliteration 'Jabir' (the letter 'a' should, correctly, have a macron over it but myLot - for reasons best known to itself - can't reproduce it!). The older form, 'Geber' is sometimes used when referring to old texts and translations (and also to some Latin texts not by him but attributed to an anonymous author referred to as 'pseudo-Geber') for the sake of clarity. There is no 'right' or 'wrong' way to transliterate his name in Roman script. There is simply an old way and a modern way.