What would cause this??

[JMBT]

      Hello all, I have a 2012 Volvo with a CM2250, and 855k miles. I just popped the top to do an overhead. It has been about 95k since the last overhead I did. Everything up there is nice and clean. All of the cam lobes look beautiful, but on the first three sets of rocker followers, cylinders 1,2,&3 there are lines staring to form. The back three still look great. I cannot feel any of the lines with my fingernail so I don’t think they are very deep yet, but I was wondering what would have caused this? And how worried should I be??

Thanks in advance! JMBT