What could make a set of the train data more predictive than the whole train data
In absence of noise and if the difference of accuracy you observe is significant, the only reason I see is that, by luck, the distribution of the training data subset happens to be closer to the distribution of your validation data than the distribution of the whole training data.
This should not happen with truly representative validation data (i.e. with training and validation data built by random split of a given dataset).