The bigger the date range selected, the more data the system needs to look for and retrieve. To keep your system running more efficiently it retrieves 50 at a time. If the number next to Leads shows a + sign (e.g. 50+), this means only 50 are currently shown in your view. Until you load everything onto one page, the selected sort order is only sorting those 50 in your current view.
What should I do?
To view all, scroll down the bottom of the list until everything gets loaded. Or you might want to view a smaller date range at a time. For example, instead of selecting Month, try Week and use the < arrow to scroll back week by week instead. This will load smaller amounts at a time and work in more of a paginated type of view. Every club has different amounts of leads so while the Month view may work very well for some clubs, others may find a better experience using the Week view and scrolling 'backwards' instead of 'down'.