A Rhetorical use of Percentages.

I saw this headline on the BBC web site:

"A landmark report says scientists are 95% certain that humans are the "dominant cause" of global warming since the 1950s">

How accurate is the 95 supposed to be? Was that number the result of a series of calculations. Might it just as well have been 94 or 96?

I believe that people decided they were fairly sure but not quite sure and decided that 95 was a suitable number to represent that degree of qualified confidence. That is sloppy thinking and undermines my confidence in the people concerned. Sir Karl Popper said the probability of any scientific hypothesis is zero. That too was rhetoric, but his zero is a stimulating challenge to the optimistic 95%.