When either form of spider is killed whilst up against a ceiling or other horizontal surface, their drops often travel up through the blocks above them (similar to a 'glass elevator' effect). This can mean the drops end up in a cave or other space sometimes a considerable distance above the point where they died.

Is it possible that drops are being created slightly too high when a mob dies?

For all other mobs this would be non-troublesome as they are on the floor and will have some space (albeit small in some cases) above them in which the drops can materialise and still fall to the ground.

However, spiders are prone to press themselves against a ceiling and, if the drops are created even slightly above the hitbox of the dying mob, they will be partially in the solid block and will glitch upwards.