The capacitive loading of XC4000 pins ranges from 10pf to 15pf. The pins near the center of the device are around 10pf. The pins on the corners are around 15pf.Thus, the spec reads 15pf. 5pf is due to the IOB and the other 5pf-10pf is from the package and arrangement. The PLCC is actually one of the better packages (less pf loading) The PGA is one of the worst (more pf loading.)