bag of visual words -Difference between default and custom extractor

What is the difference between DEFAULT and CUSTOM EXTRACTOR?

DEFAULT: No need to define, just one line in program:

bag=bagOfFeatures(imageset1A);

Information: I used default (should be SURF) and program extracted 2439360 features from first category. I used custom extractor (detect and extract SURF)and program extracted 16649 (64 dimensional) and 16352 (128 dimensional)features from first category.

Problem: Why this difference? Also execution time is around 1 hr for default whereas 5 minutes for custom extractor.

accuracy with default is better than with custom extractor. I am not using any customized parameters in custom extractor. See below:

Content of custom extractor (Myextractor):

    InterestPoints = detectSURFFeatures(grayImage);
    features = extractFeatures(grayImage, InterestPoints);

Content of program:

bag=bagOfFeatures(imageset1A,'CustomExtractor', Myextractor);