Working days, or business days, are usually said to be 8 hours long. On an average day, you may get to work at 8:30 am and leave by 5:30 pm, having 1 hour for lunch (although this differs from one culture to another, just ask someone from Mexico and you’ll see what I mean!).
Anyway, people usually spend 8 hours at work everyday. However, this doesn’t mean that people are 100% productive in the tasks they’ve been assigned to during those 8 hours. During a normal working day, normal people also check email, make phone calls, talk to their coworkers and do other things that are not necessarily related to the tasks they are working on. As a result, working days are pretty much like soccer games: while a soccer game is said to last 90 minutes, the effective playing time is usually much less than that. Likewise, the effective working time in an 8-hours business day is lower than 8 hours.
The amount of effective hours may vary from an organization to another and from an individual to another. Organizations that keep good project metrics may have a better idea of the average number of effective working hours per day. The important thing to keep in mind here is that even when a team member may be assigned full-time to a task, it cannot be assumed that he or she will devote 8 hours per day to that task. Therefore, it makes sense to think that a task that has an estimated effort of 16 person-hours will take a little more than 2 days to complete with one resource.