I'm not 100% sure that I'm interpreting your question correctly ziyuen, but the heart of the issue in the passage itself is that the fund-raisers' high success rate "does not indicate that they were doing a good job" because they've (presumably) focused their efforts on "those who have donated in the past." In other words, the passage alleges that the fund-raisers have mostly contacted potential donors who have given at least once -- and that the fund-raisers have mostly failed
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