Dynaco ST 70 bias change

I have a Will Vincent ST 70 @5 years old.  I am running KT 66’s.  Right channel tubes are 9 months old and left channel are about a year.  Checked bias this morning (routine) and found newer tubes at 1.35 and left channel at 1.15.  Last check was 3 months ago with both sides at 1.35.  My understanding is that with wear/use bias will increase.  I’m not a tech person (wish I was).  Should I be worried about that drop?  It’s always been stable for me.  Sound through Cornwall’s is lovely.  I guess I’m worried it might indicate something failing “inside” the chassis.  I appreciate any input from the knowledgeable crowd here.  Perhaps my interpretation is ass backwards too.