First, just deleting an app, can leave a lot of maps and files such as plist's that take up space, or can be incompatible with new versions of the program (I had such a problem with illustrator). I recently found that an app like AppCleaner can locate a lot of them, but still you need a good search app such as EasyFind to locate them all and remove them (Spotlight is of no help ).
Second, the installations section of systeminformation, displays all installations you ever did on your system, not only the recent updates, regardless of what is still on your system.
So I hope this clarifies my point, but as I mentionned I would like to close all discussions that are not directly related to my original question.