I've seen and understand that Acid Arrow doesn't go against Spell Resistance, as shown here. But I have no clear idea for a basis of why this would be the case? Even if you were flinging the dart that is your focus, why wouldn't the magical acid be set against SR? You are not, for the record, the text of the spell states you conjure an arrow.
A creation spell manipulates matter to create an object or creature in the place the spellcaster designates (subject to the limits noted above). If the spell has a duration other than instantaneous, magic holds the creation together, and when the spell ends, the conjured creature or object vanishes without a trace. If the spell has an instantaneous duration, the created object or creature is merely assembled through magic. It lasts indefinitely and does not depend on magic for its existence.
The acid isn't actually magical. It's created via magic, and its existence is dependent on magic, but it is, itself, actual, real acid. To further hammer this point home, Spell Resistance has this to say about Conjuration spells:
These spells are usually not subject to spell resistance unless the spell conjures some form of energy. Spells that summon creatures or produce effects that function like creatures are not subject to spell resistance.