There are several reasons why this is. The original source of the ringtone might be a quiet version. The up-loader may have scaled back on the strength of the volume when uploading. Original ringtones are put there by the manufacturer and are often optimized for the phone (in order to show the phone features off to consumers).
Remember you can always use an audio editor to increase the volume on any of your ringtones, so long as you can get them onto your PC and re-upload them to your phone. Be careful however an increase in volume does not mean in a direct increase in quality.