The 25% refers to the percentage of each class that may select USMC. It does vary year to year, but this has been the amount, more or less, for the past few classes. At one point, they thought it would approach 33% (double the 16 2/3% limit in my day). However, cutbacks in the size of the USMC have limited the growth of the USMC office corps and thus the number of selectees out of USNA.
As detailed in posts above, there is much more of a selection process today than there was in my day. This helps ensure that people selecting USMC have the ability, aptitude and desire to succeed and conversely weeds out those who are unlikely to do well at TBS and beyond.
So, if you want to be a Marine and do well in the "preliminaries," you do have a great shot. But those who are hung-ho on USMC should, of course, have a Plan B that involves the USN.