Appreciate some input on spark plug reading

I'll say it again: I can change 1 variable and it will make the plugs read REALLY RICH when I haven't changed the AFR at all. The can go from Tan before to Black after and I haven't touched the carb.

Its still metering the same Air and Fuel mixture but its now Rich.......Or is it?

Name three of these variables and then tell me which one would have the most profound effect.