Assessing multiple turbines or complete wind farms with full dynamic balancing procedures would be literally a waste of money since on average only about 10 to 20% of the turbines can or should be expected to be above the given limit of unbalance.

Single testing is the process of testing multiple turbines at once. There is no statistical benefit of just testing 10% of a site representing the condition of the site. Each turbine is statistically completely independent from the other turbines. 
Single testing is the application of only a part of a full balancing process. There is no blade angle assessment and only one test run is involved. Instead of one turbine receiving the full process of dynamic balancing in about 2 days, single testing allows for 3-5 turbines (depending on type of turbine) in one day. However, to define a baseline for comparison 3 or better 5 turbines need to be tested fully. 
As a result, the data gathered can identify the level of unbalance as well as potential aerodynamic deficiencies. Problematic turbines can then be addressed individually when needed.