payable days

What is the reason behind payable days remaining constant even when sales, cost of sales have been decreasing for three consecutive years? for instance the payble days for year 2 and 3 remain constant? how do i explain it? year 1 year 2 year 3 sales 77119557 79000314 63687754 cost of sales -33928169 -37556552 -36011992 trade payables 9,602,383 8,059,754 7,654,365