READERSHIP VERSUS CIRCULATION

I have always been somewhat skeptical of magazine and newspaper readership figures versus circulation statistics. The general assumption made by many publishers is that every copy of their print publication is read by three people. Today I witness that for myself.
I was on a short sky train trip earlier today. I noticed one man reading a copy of The Bangkok Post. As he got off he passed the newspaper to the person sitting next to him, who started to read it. Two stops later that guy got off, passing the newspaper to the passenger next to him as he alighted. That's three readers in the space of ten minutes.
I wonder how many sets of eyes saw that particular copy?