Usually battery. If the standard config is for a 6-cell battery and you get a 9-cell upgrade, that will add more weight. If there are options for bigger screen sizes, that could do it too. Really any alteration from the base spec could potentially alter it and if they didn't have that disclosure in there somewhere, someone might decide to sue because they do a lot of traveling and every gram counts, so the ad said it would be this or that weight and it ended up being 2.11kg instead of 2.10kg. It seems stupid and trivial, because it is, but people do sue over that sort of nonsense. It's also technically a truth in advertising violation that most western nations have if they don't include that disclaimer.
By and large, you can ignore it unless it makes a difference to you if something weighs 2.11kg instead of 2.1kg. For those of us who just want to know that it weighs "about 2kg" who cares?
I am buying hp Probook 440s, and the specs page mentions the weight 2.10 kg, It says the weight will vary by configuration.. What configuations is it talking about??