its clearly stated right there. their armor goes up/down based on minion presence.
also your own armor penetration and any factors/items that cause your armor penetration to change will change the amount of damage inflicted.
there are also other factors that can change the damage since they have things coded into decimal's therefore auto attacks can fluctuate slightly based on the actual damage inflicted. we are only given the solid number ex Turret HP=1500. But our damage is actually into the 100ths or X.YY and a few attacks will actually change your numbers you are being shown because we are only granted a visual of WHOLE numbers like 12 or 15 when in reality we may be doing 12.13 or 15.76 damage. So all of this combined can give you a bunch of different variables. We are not given an exact distance that minion presence becomes a factor, they left that kind of information out of any explanation/description. So you may see turret info fluctuating but it's just due to things have X.YY coding and only a visual representation of ~X. Check out some champs and masteries along with items in late game and you may notice that your own resistances fluctuate even when standing still out in a bush with nothing around. it works the same way with homeguard boots. recall once with homeguards on with no one around.... you see that your runspeed is fluctuating? that's because it is based off of a percentage yet we are only given the whole number without decimals as visual representation.
I hope that this has cleared some of those fluctuations you may be noticing, but it is not a bug and it's working as intended. The coders/programmers just don't give us the minute details as it's really not needed for us to play the game, what they give us is a frame of reference so that we can see what is being affected and by how much more or less. Going deeper into the decimals of champion/ability/item/map statistics is way too much information that we really don't need access to. The flat numbers give us all we truly need to do what we want with the champions.