Mfg recommendation might have been determined using different tubes than you're using. Even same brand tubes can vary between batches. Different brands of same tube will behave differently, especially new production versus vintage tubes of same type. And of course you will have a different system and taste than the amp designer, and might not need or prefer the edge/presence that higher bias gives. Bias current is set to nominal current regardless of voltage variation which is usually slight. Bias will still fluctuate with voltage changes like refrg turning on, etc, but that's expected. But it is always safe to turn it down to taste, but you might blow a fuse if you crank it up too far. Turning down the bias current will also prolong the tube life. Sounds like you found a win/win. I have my tubes turned down a just a couple mA too. They are less fussy.
