Mosin-Nagant Stock Weirdness?

[Zeke Menuar]

I have a re-arsenaled Russian M38, 1944 mfg date. I recently used denatured alcohol to strip off the shellac. I replaced the finish with four light coats of satin varnish and two coats of Johnson's paste wax. I assumed that was that and I went on my merry way. In the three weeks since I finished the job, the stock is becoming darker. Much darker. Why is this happening? I am not disappointed. It looks great. But I would like to know how this phenomenon happens.

Thanks
ZM