A credit purchase of inventory increases both current assets (inventory goes up) and current liabilities (accounts payable goes up) by the same amount. When the ratio is $> 1$, adding equal amounts to both numerator and denominator brings the ratio closer to 1 — i.e., it decreases. For example, if CA $= 200$ and CL $= 100$ (ratio $= 2$), adding 50 to each gives $250/150 \approx 1.67$ — lower but still above 1. So the ratio decreases but stays above 1.