In order to provide new light on the deep debate about discrepancies between world University ranking results, an attempt is made to compare them on a rigorous quantitative basis, using an original methodology. Three well known rankings, namely Academic Ranking of World Universities (ARWU), Times Higher Education (THE) and Quacquarelli Symonds (QS) rankings are compared by means of ternary and binary diagrams for their 2010-2011-2012 editions. Four countries or culturally consistent sets of countries are analyzed: the United States, the United Kingdom, Northern Europe (Germany, Netherlands, Scandinavia), Southern Europe (France, Italy, Portugal, Spain). The most established ranking, ARWU, being taken as the reference of activity in research, it is observed that the THE and QS rankings globally underestimate universities best ranked for research and overestimate universities less well ranked for research. More specifically they underestimate the US, Northern and Southern Europe, and overestimate the UK universities. These observations just point out that reputation surveys, which represent a very significant weight in THE and QS rankings, could be the most relevant-and puzzling factor to explain for the overall differences of results between the three rankings.