It should be an easy thing to talk about - when a guitar is in tune, that’s it - right? Well, sort of, but if you use a tuner, think again. 

I think that modern day digital tuners serve to reassure us that everything is ok, to give us confidence so that we don’t spend ages twiddling those little knobs. It’s modern technology - it must be accurate! Actually, they are just a useable compromise for tuning quickly before a performance, certainly not good enough to judge intonation with. 

Buy yourself an analogue tuner, one with an actual moving needle on it. Play the note and watch the needle. It will not stop moving for about 4 or 5 seconds. If you listen carefully while you watch, you will notice the pitch of the string changes in step with the needle. The changes are significant, certainly enough for you to query your tuning.