Dúvidas

#include <stdio.h>
#include <math.h>
#define PI 3.141592654

int main(){

double a = 0;

while(a<=2*PI){
printf("cos(%e) = %e --- sen(%e) = %e\n", a, cos(a), a, sin(a))
a = a + PI/2;
}

return 0;
}

esse programa me retorna

cos(0.000000e+00) = 1.000000e+00 --- sen(0.000000e+00) = 0.000000e+00
cos(1.570796e+00) = -2.051034e-10 --- sen(1.570796e+00) = 1.000000e+00
cos(3.141593e+00) = -1.000000e+00 --- sen(3.141593e+00) = -4.102069e-10
cos(4.712389e+00) = 6.153103e-10 --- sen(4.712389e+00) = -1.000000e+00
cos(6.283185e+00) = 1.000000e+00 --- sen(6.283185e+00) = 8.204137e-10

alguem tem alguma explicação!? procurei na internet algum erro relacionado a isso, mas sem sucesso...