you are placing the pickup under different nodes along the string meaning that the harmonic response of the pickup and the tone are different.
i have never believed all this talk of nodes. For a start a pickup is too big to actually detect a node point acturately - especially a humbucker... and if it could do it then surely the string would be almost still in that point and it would only be detecting the harmonics
what you can see quite easily know is that a string moves differently along its length - take a plectrum and pluck the string right next to the bridge, hardly makes a sound, move a mm and try again... after about 1/2-3/4" you will notice a huge jump in the response of the note - less brittle and more rounded.
the closer a pickup is to the bridge the brighter it sounds, obviously you can tilt a pickup and it will be different than straight because of this.
this talk of nodes normally comes along with the 22-24 fret debate. 22 fret guitars are said to have a better neck pickup because its usually under the 24th fret node.... or is it simply that its further away from the bridge one and gives more tonal variation than the squashed together pickups on a 24 fret guitar