Why do effects that exile a card from the graveyard use the characteristics the card had in the graveyard?
There is a flaw in the basic premise of the question that these are conflicting rulings on comparable cards.

Deicide has an effect that moves an object to another zone and then further examines that card.  This is what 'the expected zone' clause in 608.g covers.  There is consistency in the rulings when an effect moves a card between public zones and then checks types using the present tense, it looks in the new zone.  (See Defy Death). As opposed to an effect that moves a card between public zones and checks its type in the past tense (lots, Death's Caress). It seems that does not have particularly clear basis in comp rules without rulings clarifying the intended interpretation.

Dread Defiler has an effect that looks for characteristics of an object.  Paying the cost for the ability that generated the effect moved that object, but the effect did not move the object.

So the two rulings are not in conflict.

Soul Separator is not doing a conditional check, so, assuming the ruling on it is correctly worded, that seems to be our plausible explanation.  If it said "if that card is Power 5 or greater" we'd be in a pickle for which zone to check in I think...

I agree it is entirely possible for a rational person to have come to different conclusions from reading the comp rules and cards in isolation.  Sometimes a card like Deicide with odd nonstandard templating just needs the guy who wrote it to explain what they intended.  (The use of 'God card' rather than simply 'God' combined with two public zones makes it close to unique.  Almost everything that checks the type/subtype of a "card" rather than a game object and involves a zone change also involves a hidden zone)