It depends on the year. In 2009, there wasn't anything conceptually challenging on the paper, so it didn't discriminate between students at higher levels. That said, 50s in any subject are only given to approximately 0.2% of students, so you've still got to beat 99.8% of the cohort.
In other years though, the papers have been more difficult, and hence you've had to have a pronounced talent to succeed. I do wonder whether a lot of the top scorers from this year would have achieved highly anyway - I know that at my school, 2 of the 3 50 scorers attained 50 in Methods, including one who'd also 50-ed Chemistry (neither of them had rank 1 in physics, but they were both smart girls).