Well there's a multiplicity of ways of looking at the decans (faces). From your post I guess that the 'system' that is refrerenced is one of keeping track of time during the night.
Keeping track of time during the day is reaonably easy (allowing for decent weather conditions) as the Sun's movement defines both night and day and also morning and afternoon - add a sun dial and you can be quite precise. The problem of night is that there's no sun (by definition) and there can even be no Moon.
It appears that the Egyptians used rising stars as a way of solving the problem. They used star patterns (asterisms/constellations) as easily identifiable markers and seem to have used 36 such constellations which on average rise 40 minutes apart. Now these constellations do not lie on the zodiac alone but seem to have been chosen for easy recognition. Therefore they rise at various places along the eastern horizon.
The twelve signs of the zodiac are solar based if only for the obvious reason that the ecliptic (on which they lie) is the apparant movement of the Sun around the Earth. So it is possible to cross reference the two systems of measurement and assign one of the 36 constellations to a segment of 10 degrees of each Sign.
Now I'm no expert on Egyptian or Babylonian Astrology, so my suggestions for variations between systems are very tentative:
Firstly it seems that 'system' perhaps over dignifies it. It may have been more a matter of convenience and as such may have changed over time.
Secondly the Egyptian 'system' was not universally used in classical times
Thirdly precession changes the relationship between tropical signs and asterisms. So what was the case in say, 400 BC is not the same today, indeed it was and is constantly changing.
A good article is :
http://www.nyskies.org/articles/pazmino/decanate.htm
Hope that helps - incidentally one point made in the article is the lack of any clear (or intended?) interpretations for the decanates in this approach.
