I own both ANAT and NWLI, so I have known of ANAT's strong stock price performance. Actually, it has been much better than you have said if you go back farther. I bought a good bit of ANAT in October 0f 2012 at around $65. The market price was way too low then. I think NWLI's stock price is still way too low now and I have been buying more of it.
Might be also because ANAT has a large P&C business - about 2/3 of premiums. Pretty much all the P&C companies have done better than the life companies and trade at higher valuations. They are seen as being lower risk as most of the insurance they sell is shorter term, so they can adjust their rates faster to reflect new claims conditions, but also the new low rate investment environment.
I think lifeco's are in the sweet spot now though with better valuations plus interest rates starting to move upwards.