Why are 'Geser' and 'Zavulon' transliterated to Gesar and Zabulon?

Question

In the original Russian version of the Night Watch series, I believe the heads of the Moscow Night and Day Watches are called Гесер and Завулон respectively. (I don't actually have a copy of any of the books in Russian, but I've picked up this information from DVK's answers about the books, in which he includes both Russian text and his own English translation.)

In the English translations I've been reading, the names are rendered as Gesar and Zabulon, although a more literal transliteration of the above names written in Cyrillic would be "Geser" and "Zavulon". What is the reason for this deviation from a letter-by-letter transliteration? As far as I can tell, "Gesar" and "Zabulon" are no easier to pronounce in English than "Geser" and "Zavulon", nor is there any hidden meaning in any of these names which is apparent to the English-speaking eye.

Why were their names translated to Gesar and Zabulon rather than Geser and Zavulon?

Answer 1

NOTE: 2018/02 - updaed the answer with more WoG/canon info on the name origins.

Zabulon vs. Zavulon:

The origin of the name is Old Testament as per the author's off-line interview on rusf.ru; after the 10th son of Jacob:

Происхождение имени Завулон... Это один из двенадцати сыновей Иакова, последний из шести сыновей, рожденный Лией, родоначальник-эпоним одного из "колен Израилевых"...

The origin of name Zabulon... He is one of the twelve sons of Jacob, the last of the six sons born by Leah, the eponymous progenitor of one of the twelve tribes of Israel

English versions of Old Testament/Bible spell the name of 10th son of Jacob as Zebulun, using a typical for such transliteration latinized/English "B", and therefore the translators were indeed correct in a way by using "B" instead of "V".

Please note that this is a well known language pattern (albeit in reverse): you see a Christian Biblical name with a "B", which is translated into Russian biblical text with a "V" (possibly due to Greek translation). For example, Nebuchadnezzar got turned into Greek Ναβουχοδονόσωρ, and Greek β maps out to Russian "в" which is a "V" not a "B", resulting in Russian name "Навуходоносор".

Geser vs Gesar:

The origin of the name is of Mongolian and Tibetian hero/god (Гэсэр) - as discussed at length in my answer here.

And English version of the mythological name is indeed Gesar.

I'm pretty sure either "a" or "e" version is plausible since the phoneme used in original mongolian is listed as "ə" which can be transliterated as either one.

As a note, the name is ambiguously transliterated even into Russian - I have seen versions of "Гесэр", "Гесер" and even "Кесар"