because frequency, like intensity, is logarithmic in nature not linear - a doubling in frequency is the same as making it an octave higher and vice versa
200Hz is one octave higher than 100Hz and 1 octave lower than 400Hz and 2 octaves lower than 800Hz and 3 octaves lower than 1600Hz if you look at the distance between each of those frequencies in a logarithmic scale they are roughly equal
when we look at the relationship of these 5 frequencies we also can see that we are talking about
100Hz x 2[sup]0[/sup] = 100Hz x 1 = 100Hz
100Hz x 2[sup]1[/sup] = 100Hz x 2 = 200Hz
100Hz x 2[sup]2[/sup] = 100Hz x 4 = 400Hz
100Hz x 2[sup]3[/sup] = 100Hz x 8 = 800Hz
100Hz x 2[sup]4[/sup] = 100Hz x 16 = 1600Hz