Why does multiplying two fractional Doubles and converting the result to a String result in extraneous zeroes?

Question

var d1 = 0.124 * 0.12456 // 0.01544544
var s1 = String(d1)      // 0.015445440000000001

var d2 = 0.01544544      // 0.01544544
var s2 = String(d2)      // 0.0154454