שמואל א, פרק י״ז, פסוק ג׳

The deployment of forces creates a tense standoff, with both armies facing each other from positions of advantage. This choice of location is deliberate; in times of war, military forces naturally try to take commanding positions on high ground [ביאור שטיינזלץ]. The Philistine warriors advanced right up to the mountain across from the Israelite forces, leaving only a valley to separate the two armies [מלבי״ם].

Although the Israelites were previously said to be camped in the valley, there is no contradiction with their current position on the mountain. The entire main camp was indeed situated down in the valley, but the actual fighters and those preparing for combat went up to secure positions on the mountain to face the Philistines [רד״ק].