Simply because the solution to your sum produces a repeating decimal. Just as 22/7 (The value of Pi as a fraction) produces the repeating decimal 3.142857
What does the "why" question refer to, specifically? ANY division you do, of one whole number by another, you will always get either a terminating decimal, or a repeating decimal.
1.625
39 / 4 = 9.75
9.6662
6.33333 repeating or rounded to 6.3
5.5714
-39
4.3333
19.9286
Equal to it 39 divided by 20 is 1.95. It is probably quicker to use a calculator than to ask on line
2.5641
19 divided by 39 = 0.48717948717948717
39 divided 3 = 13