Toggling between the List View and Map View on the Search Results page can change the number of properties that are displayed. When you switch between the two views, you'll notice the bounding box of the map changes. When results are returned using the Map View, a perimeter around the area you are searching is included in the map and therefore in the search results. Often you will receive more results when searching in this view. When you switch to List View, the search no longer includes a buffer area outside the area you are searching, and therefore the number of results may be lower.
To avoid this effect, you can select the "redo search as map moves" checkbox above the map. As you pan on the map, your search will dynamically shift and reload to display results that are congruent with your map view.