Why do some pins designated as differential in Cyclone® V, Arria® V, and Stratix® V devices have no complimentary pin?

Description

The differential pin index is usually determined from the largest package of a particular device density in Cyclone® V, Arria® V, or Stratix® V devices. Hence, some differential pairs might not exist for some smaller packages for a given device density.

For example, pin DIFFIO_TX_T18p might exist, but DIFFIO_TX_T18n may not exist in a device pin-out for a particular package.

Resolution

Differential pins that do not have their complementary pair cannot be used for differential functions. Only use pins that are available as a complete pair for differential functions. 
The differential pin index is usually determined from the largest package of a particular device. Hence, some differential pairs might not exist for some smaller packages within the same device.