This seems an odd way to do this, but apparantly there is no universally
accepted method for quartile so you can't say Excell is incorrect in doing it
this way. I copied and pasted the following from
http://www.staff.city.ac.uk/r.j.gerr...elfaq/faq.html

If the data are assumed to be in ascending order,
- the lower quartile (Q1) is the ¼(n+3)th observation
- the second quartile (median) is the ½(n+1)th observation
- the upper quartile (Q3) is the ¼(3n+1)th observation
When ¼(n+3), ½(n+1) and ¼(3n+1) are not integers, linear interpolation is
used.