Think of it this way. Say you've set the extent so the X difference is 100,000 units. You've set the size of your map to be 1 unit wide, which makes the scale of your displayed map 1:100,000. If the tiled layer is set up with the Web mercator standard levels of detail at 1:144,447.638572 and 1:72,223.819286, the extent of the map will have to change to fit one of those scale. When you're using tiled layers, you cannot set arbitary scales, but have to use the pre-determined scales. If you're using only dynamic layers, you can set any scale you'd like.