BGII Multi-Class HP?

[Haucusuchus]

How is this calculated? I recently made a fighter-mage with 18 Constitution, and I noticed that he would ALWAYS gain 7HP on a fighter level, and 4Hp on a mage one . Since 18 CON gives a +3 bonus to HP (I think), that would bring the base HP acquisition to a sad total of 5 every two levels. Whats up with this? I thought HP acquisition was supposed to be random, and that multi-class characters would use the hit dies for each respective class .

[JPS]

This used to bother me a bit when I was playing BG 1, which doesn't have a difficulty setting that gives you the highest possible roll. I never knew when to reload

 

From the 2nd edition AD&D Player's Handbook:

 

The character's hit points are the average of all his Hit Dice rolls. When the character is first created, the player rolls hit points for  each class separately, totals them up, then divides by the number of dice rolled (round fractions down). Any Constitution bonus is then added to the character's hit points. If one of the character's classes is fighter and he has a Constitution of 17 or 18, then he gains the +3 or +4 Constitution bonus available only to warriors (instead of the +2 maximum available  to the other character classes).

Later the character is likely to gain levels in different classes at different times. When this happens, roll  the appropriate Hit Die and divide the result by the number of classes the character has (round fractions down, but a Hit Die never yields less than 1 hit point). The character's Constitution bonus is split between his classes; thus a fighter/mage gets 1/2 of his Con bonus when he goes up a level as a fighter and the other 1/2 of the Con bonus when he goes up a level as a mage. A fighter/mage/thief would get 1/3 of his bonus when he goes up as a  fighter, 1/3 when he goes up as a mage, and the other 1/3 when he goes up as a thief.

 

So a fighter/mage with 18 constitution would get [(10+4)/2]+4 HP at first level and after that 10/2+4/2 HP for every figter level and 4/2+4/2 HP for every mage level (that's supposed to be 9, 7 and 4, in case I got the maths wrong...). I never managed to figure out why it was done this way, but it's a good argument in any 2nd ed vs. 3rd ed debates, so I'm not complaining