Hi Gianluca,
The way paste works is as follows: The clip(s) to be pasted will start from the lowest numbered track that has Auto Track Selector on.
If one clip is pasted, it goes to the lowest numbered Auto Selected track.
If two or more clips are pasted, the first goes to the lowest numbered Auto Selected track, the second and subsequent go above that in the same relation as it was in the copy source.
So if you copy two clips from tracks 3 & 4 and paste into a timeline where Track 2 is the lowest Auto Selected, the clips go to Tracks 3 & 4. If you copy two clips from Tracks 2 & 5 then paste into a timeline where Track 3 is lowest Auto Selected, they will go onto Tracks 3 and Tracks 6.
In all cases, it's the lowest numbered Auto Track Selector that decides the starting track. A key word here is 'lowest'. In your example it sounds like you disabled the Auto Selector on Track 2, when in fact what you wanted was to disable the Auto Selector on Track 1, so the clip would go to Track 2.
If no track has an Auto Selector enabled, the clip(s) start on a new track above the current highest.
Please see this short example video I made that should demonstrate the permutations:
Note also that Track Auto Selectors are also used for some other functions, such as the behaviour of the Next/Previous Clip/Edit shortcuts (Down/Up arrow respectively, by default), so they are quite important and powerful. There are keyboard shortcuts to toggle them for individual video or audio tracks, or all video or all audio tracks at once.