Those are Instant Commits. A player can instant commit when you are #1 on his board and you offer him a scholarship. Lower-caliber players and players are more likely to instant commit than higher ones, so traditionally the only lower-caliber guys I recruit are guys who have me ranked #1. It's also possible for players to instant commit in later weeks (if you offer a scholarship then, or have already offered one), but most occur in Week 1.
That's why I try to put 16-18 guys on my board each preseason. The odds are that a player will instant commit to you, and a couple will go to other schools in Week 1, leaving you with a healthy 14-15 players on the board. I try to schedule visits around Week 7 or 8, or whenever I feel like I can get a soft commit.
Pay attention to how much your recruting is actually working. Usually I won't recruit someone from the start when I'm more than 300 points behind first place. If you recruit from a large deficit and notice the 1st place team is gaining interest at a freakish pace, you WILL NOT get the guy, even if you spend 60 minutes on him each week. Just drop him off the board. Spending 40 minutes on one player at a position is better than 20 minutes with 2 players at that position. Sometimes you are the team moving at that freakish pace, so you can afford to use less minutes on that guy and more elsewhere.
If you're using a high-prestige team, check the Top 150 again after the first 2-3 weeks. There are many players (usually WR, HB, and ATH) that no other teams have pursued at all. Those players are very easy to get.