Housing Benefit and Sub-tenants

Hi folks

I was just playing with my council's benefit calculator and am really surprised at results. First, I put my details in and added another person as a joint tenant. Result, I would get LHA shared room rate. No surprise there.

Then I put in the exact same details but changed the other person from joint tenant to sub-tenant. I thought the result would be the same, but it's actually more.

Can't find the rules on this. Does anyone know? Seems very odd.