Hmm I suspect you may be adding 2+2 and coming up with 7 or comparing apples to oranges. I have heard One-Eyed Jack state that buffs do stack in this thread. So stacking your strength should and will increase your guys. There are other factors that may be suppressing the criticals including just the roll of the dice in the chance factor.
Ratbeard would be the expert in helping you to understand what factors tweak when and how often this happens.

I think relentless is supposed to be random. I think that one had a die and a percentage on it's description, meaning it's a chance draw how often it goes off. If it seemed to be happening more when you improve strength, that might be coincidence. I'm thinking both relentless and strength buffs both happen often enough, that it's not unusual for relentless to trigger while strength is improved. You probably don't get to do multiple strength improvements at one time as often, and you hit some rolls that didn't give relentless.