I don’t know what the “standard” data split for EMNIST-Letters would be, but it seems you are checking the highest validation accuracy, which might be invalid. In a common setup, you would split the data into a training, validation, and test dataset. The training and validation splits are used to train and validate the model, respectively, while the unseen test set should be used once after the training is finished (e.g. after you’ve used early stopping using the validation accuracy).
If you are using a two-way split, one could argue that the validation data was already “seen” or “used” in the training process, since you are using the validation accuracy to store the best model.
In any case, I don’t know if there are predefined training, validation, test splits or how e.g. this benchmark 2 on paperswithcode was created.