Per the docs, justify_days is to "Adjust interval so 30-day time periods are represented as months" so 360 days = 12 months = 1 year so 365 days is 1-year 5-days.
There are all sorts of oddities and special assumptions regarding date/time calculations made even more complicated by the need to support special use-cases such as 30/360 financial coupon factor calculations (every month is 30-days and years have 360 days).
Cheers,
Steve