Hello Aymen
When configuring static object NAT, you must define the translation for both the source and destination addresses. For example, take a look at the following diagram (ignore the fact that this is a router and not an ASA, the principle is the same):
When you configure object NAT, you configure the translation between the source addresses as well as the translation between the destination addresses. Notice in the diagram that it is the source address that changes, that is translated. The destination address (in this example) remains the same.
So when you configure object NAT, you are translating between Object-group_Private and Object_group_Public which are different (hence the translation that occurs. But for the destination, you are translating between Object-group_1 and Object-group_1 which are the same. They are the same because you are stating that no translation should take place.
You can find out more information and more examples at the following Cisco ASA command reference:
I hope this has been helpful!
Laz