"Master" status among vampires runs along the same principles as physical prowess in humanity: not everyone is the same. We have athletes in pro sports, the Olympics, etc., who can do things physically that most average people cannot. With Master vampires, they can do things that most average vampires cannot, like fly, be stronger, or even have special abilities (mind reading, telekinesis, pyrokinesis, etc).
What makes a vampire a Master? Sometimes lineage can help, but mostly, it has to do with the individual. Everything a person was when he/she was human gets amplified when becoming a vampire. A weak-willed person who becomes a vampire will likely not become a Master, because there's not as much for the supernatural to draw from. A very strong-willed person who becomes a vampire will have a greater tendency to become a Master, especially if that person was sired by a Master. A very physically strong person who becomes a vampire will probably be stronger than the average vampire - unless that vampire is a Master. A person very strong mentally and physically who becomes a vampire, sired by a Masters (or from a long line of Masters), has a much higher chance of becoming a Master vampire. The traits of being a Master usually show up in the first fifty years of being undead. After that, power can grow with age, but not at the accelerate rate of being a Master, or with the extra perks (flying, mind reading, telekinesis, pyrokinesis, dream-snatching, etc
And sometimes, it's just a fluke, but that's very rare.