It says it is not representative *of all*.
But that it is a reliable indication of the global trend.
If you sampled 1,000 Americans at random, it would be a reliable indication of the obesity problem, but it might not represent the anorexics very well. You might need a sample size of 10,000 to represent the prevalence of American anorexia.
Same deal here. All the sampled glaciers are melting at an increasing rate. This does not mean that every single glacier on Earth is melting at an increasing rate but it captures the global trend very well. A larger sample than the number of glaciers sampled would be necessary to asccurately state the global trend at which the rate of melt is increasing. This only estimates it.