Double tap to wake?

I will wait to update, waiting to if see a new firmware comes along soon to enable the option and fix some issue. That I am seeing since gdr2/amber ( lockscreen not responding correctly just lock's sometime have to hit power button to get it to respond,

I updated display and touch the other day, since then I have not had the issue above.
 
What is the reason why double tap to wake is on the 520 but not the 521. "Hardware limitations" although they both have the same hardware?
 
What is the reason why double tap to wake is on the 520 but not the 521. "Hardware limitations" although they both have the same hardware?

I think its a AT&T exclusive. The non-AT&T 520 is missing it as well. Oh well, at least the 521 has the extra camera sensor and wifi calling.