The default I/O Threshold displayed is based upon the I/O Standard. If you want to set the I/O Threshold independently of the I/O specification, you must select CUSTOM in the I/O Standard cell.
To specify I/O Threshold:
Select CUSTOM from the I/O Standard cell in the macro row you want to edit.
Select CMOS, TTL, or PCI from the I/O Threshold cell. CMOS should be faster on high-to-low transitions, and TTL should be faster on low-to-high transitions.