I'Ve Noticed that Kog'maw randomly doesn't proc phantom hit every 2nd attack when on his W.

I just played a bots game to test out the new rageblade on a full on hit build kog'Maw ( so that he will get insane phantom hit procs ) And noticed that while on a fully stacked rageblade i would always proc phantom hit every 2nd auto like normal, but when i turn W on it randomly becomes every 4th hit or 2nd hit or sometimes even 6th hit... Is this intentional ? If