Workforce Engagement Management

When agents submit a time-off request, the system uses the agent's Work Plan to estimate the total paid hours for the date range the user requests (if a published schedule is not present). How does this work if there are optional days in the agent's Work Plan? From the very limited testing I have performed it seems to be very random, sometimes it will set the optional days to include no paid hours but other times the optional day does include paid time.