This study explores the variation within native English-speaking teachers (NESTs) when evaluating English learners' essays in terms of their teaching experiences. It was assumed that those who had been teaching at the same institute for more than five years (experienced NESTs) would understand better what their students wanted to convey, with deeper understandings of Korean culture and possibly the different writing styles based on Korean language, resulting in more lenient evaluating patterns, than those who just came to Korea to teach English (new NESTs). Interestingly, in contrast to the previous studies, the experienced NESTs evaluated more severely than the new NESTs. Additionally, based on the previous studies comparing NESTs with non-native English-speaking teachers (NNESTs), the experienced group demonstrated similarities to NNESTs after more than five years of assimilation in contrast to the new NESTs. This was a conclusion reached by the result that the new NESTs regarded content more substantially than the experienced NESTs. Lastly, unlike the previous studies, the NESTs' perceived-difficulty played a more important role in grading than their perceived-importance regarding the three criteria (grammar, content, and vocabulary). The paper elaborates the details of the results and suggests a new approach to reduce the rater-biases even before rater-training is discussed.