How BenchmarkPi works!
So, for all of you that does not know, Pi or π is a mathematical constant whose value is the ratio of any circle's circumference to its diameter in Euclidean space. π is an irrational number, which means that its value cannot be expressed exactly as a fraction m/n, where m and n are integers. Consequently, its decimal representation never ends or repeats. It is also a transcendental number, which implies, among other things, that no finite sequence of algebraic operations on integers (powers, roots, sums, etc.) can be equal to its value.
You can find more information about π at Wikipedia.
The whole idea behind this application is to use the same Pi calculation algorithm on every Android Device and check how fast that proccess is. Better calculation times, conclude to faster Android devices. This way you can also check how lightweight your custom made Android build is. Or not.
As Pi is an irrational number, Benchmark Pi does not calculate the actual Pi number, but an approximation near the first digits of Pi over the same calculation circles the algorithms needs.
So, the number you are getting in miliseconds is the time your mobile device takes to run and complete the Pi calculation algorithm resulting in a approximation of the first Pi digits.