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.
Nothing is failing in the chassis- both sets of tubes run off of the same bias supply. You simply have power tubes that are getting weaker.
Thanks for that.  I’m retired, and the amp is on quite a few hours most days.  Cost of doing business.