Not promoted at work?

The job I applied for is a dept. manager job at a retail store. The other candidate had been a dept. mgr at another location for about four years, so it would be a lateral move for her, but it would've been a promotion for me. I am a head cashier (for about one year). My boss called me to his office to tell me he gave the job to the out of state applicant from another store. He gave me a lot of praise and compliments. Our company uses a scoring process in interviewing. He said that although she has been doing the job for four years, our interview scores were exactly identical. I scored as well as someone with four years in the job. BUT he gave the job to her, he said he was being selfish, because he said that way, he fills the vacancy with a great employee AND keeps me as a head cashier. I feel betrayed. I don't understand why he didn't promote HIS STORE employee, someone he knows, trusts, counts on every day, who has excellent attendance and punctuality. I just don't get it! I am really working hard to just carry on and not let it affect me. But, it's just so disappointing.