With respect people are not missing the point ..it is merely that you have just noticed something that many of us have been aware of for a long time ..including the +1 effect..Not everyone is wondering why they rank as they do .and +1 does not explain all of it..
Nor does everyone use such simplistic methods of determining their ranking either from different IPs or from those who are signed in or out of Google properties..
It is personalisation, whether or not you personally put it in that category is entirely dependent upon your ( apparently ) very narrow and simplistic definition of personalisation ..
As to why it happens ..the reasons have been given many many times here ..maybe you have merely missed the threads, or not put together the pieces given in them to make the picture that explains this ?
I would suspect that most people here have no clue
As Netmeg says this "info" is nothing new to many of us ..bears, popes and woods comes to mind..
And as always, the more sites one has under ones control,the wider their "niche" spread, and the more machines and distinct IPs one has available to use when researching the what and why and how of serps ..
The more one reads here and elsewhere , the wider one reads, and the less one posts..the more one understands and the less one is taken unawares and the less one is "surprised".
On a small scale, an easy way to observe what happens is to visit more than one cyber-cafe regularly and check some keywords ( nor necessarily your own )..or when you visit friends, ask to use their machines for 5 minutes, or phone and ask them to search for you and to send you screen caps ( logged in and out etc , +1'd or not )..maybe offer something in return..
Serps are like the Zen "glass of water", how it tastes depends on who you are, where you are, and how thirsty or interested in water etc you are at the time..