Slow and steady active but headshots?

Hi guys, I am new to pvp and most of time I am using Slow and steady. Today 2 times in pvp I encountered with players who use PIB and all the time I died with headshots while my slow and steady card active.(they were using lanchester) Please correct me if I am wrong but as far I understand Slow and steady prevents from headshots.Is there a point I missed? Thank you in advance for your comments!