grammaticality - ran lower to the ground vs ran low to the ground

Can you explain to me why author have used word "lower" instead of "low". The text is:

I was too far away to see anyone, so I parked behind a big rock at the base of the mountain, left my helmet there, and hurried into the tall grass. The trees were about 200 yards away when I ducked and ran lower to the ground.

Is that correct to tell "ran lower to the ground"?.