Error calculation in Logistic Regression
The problem here is that you are comparing a regression problem with a classification problem. Classifying MNIST digits is treated as a classification task because it is assumed that predicting the wrong digit is always bad, regardless of how "close" you are to the right answer. One could even argue that predicting a 9 instead of a 3 is a better guess than predicting 5 for instance.

If you want really want to compare the values, I would simply pick the class with the highest value and use this as the regression prediction.