beacuse silver has two valencies,that is ,1 and 2.So sometimes Silver with valency one combines with chlorine(Having valency 1)to form AgCl and sometimes silver with valency two combines with chlorine(valency=1)to form AgCl2.
1 4 1
This is because silver has two valencies. silver having only 1 valency is called as argentous that is Ag+. Silver having two valencies is called as argentic that is Ag2+. some elements have more than ome valency. they are said to have variable valency. Silver has variable valency. other examples are iron, copper,lead and so on