Here are 2 events, both showing arg to args.
You can see, that destFlags are missing on the first event (because it's an empty string, when you check pure Lua event), and therefore you get one more event (spellSchool as arg).
While it's possible to work around this, it messes with consistency. Would be great if it could be fixed.
timestamp event sourceGUID sourceName sourceFlags destGUID destName destFlags spellID spellName spellSchool 1500657459.841 SPELL_CAST_SUCCESS 0x00000000000066FE Secrets 1304 0x0000000000000000 -2147483648 32223 Crusader Aura 2 1500657477.524 SPELL_CAST_SUCCESS 0x00000000000168CD Chrys 1304 0x00000000000168CD Chrys 1304 6346 Fear Ward