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According to HPCT.2DA, cleric/thieves get 1d7 HP at levelup until level 9 and +3 past that.
Since rogues get d6 and clerics d8 for hit dice, the random part is correct.
However both of them get +2 hp per level at high levels, so cleric/thief should get the average of that: +2 hp per level pas 9.
Hence the +3 is a bug.
Rogues get one extra hit dice at level 10, so one might want to set that levelup as 1d3+1 and similarly tweak every other thief multiclass table. This is of course assuming I understand correctly the way the table is pulled at levelup and this change does not lead to accidental screwups....