If you have used several crossbows in a hunting situation you will find that some are slightly more quiet than others and there are things that can be done to quiet them down a bit! As DNK stated Wood stocks absorb noise. Some people fill their composite stocks with ureathane foam to deaden some of the noise. Some use vibration dampening devices, some use string silencers and STS systems. Saying you heard several crossbows being shot doesn't mean dodiddily squat.

All crossbows are noisier than vertical bows. Most shoot fast enough that at ethical shooting distances, where the deer is relaxed and unaware of the hunters presence; the noise is a moot point