Rocker shaft pitting question.

Hi,

This is a pic of pitting/scoring on the rocker shaft of my Nissan KA24E OHC I4 with 177k miles on it.  
This is the rearmost exhaust rocker arm position. All the rest of the rocker arm wear points look much better on both the intake and exhaust shafts  

Anyone know why this would happen at just one rocker arm location, while the rest are fine?  Is it lack of oil, contamination, or maybe just a manufacturing defect? 

Thank you!
 

What does the inside of the rocker look like?

Russian Warship, Go Berkeley Yourself said:

What does the inside of the rocker look like?

Similar to the shaft: Scoring and pitting, though not at bad as on the shaft.