Lanun Tricorne - What Does Phantom Roll +50 Mean?

The new reforged relic COR hat (Lanun Tricorne) has an added effect "Phantom Roll effects +50". Does anyone know what this is?

I equipped the hat and used /checkparam to show my melee accuracy as 504. I then rolled a Hunter's Roll of 3. /checkparam again, and my acc had increased to 534, or +30. Exactly what a Hunter's of 3 is predicted to give (with RNG in party). Clearly no +50 to the effect.

So I then thought that the +50 might refer to duration. Waited for my Hunter's to wear off - and no time increase at all.

Thoughts?

Considering the old tricorne's effect, I'd say it's a 50% chance of gaining the job bonus despite not having said job in the party (ie: 1 extra MP a tick from Evoker's without a SMN in party).