Mongodb – Mongoose id and _id changes the value in response

I am getting different ID value in JSON response when checking variable as id & _id in DTO

CASE I ( using id as variable )

DTO for the response

export class UserDetail {
    
        @Expose( )
        id: string; //HERE ID is used which gives original id as in db
    
        @Expose( )
        name: string;
    
        @IsOptional( )
        @Expose( )
        mobile: string;
   }

Response for this is:

        {
            "id": "6229df5cc32d3aaef32525e0", // Correct id as in db
            "name": "Name1 Title1",
            "mobile": "AB391C339",
        },
        {
            "id": "6229df7bc32d3aaef32525e3", // Correct id as in db
            "name": "Name2 Title2",
            "mobile": "CDE393F339",
        }

CASE II ( using _id as variable )

Now changing DTO

export class UserDetail {
        
       @Expose( )
       _id: string; //HERE _id is used which gives different id value from original one and subsequent ids are in incremental fashion
        
       @Expose( )
       name: string;
        
       @IsOptional( )
       @Expose( )
       mobile: string;
  }

Response for this is:

        {
            "_id": "624023c8193f2404f8312ccb", // Not same as present in db
            "name": "Name1 Title1",
            "mobile": "AB391C339"
        },
        {
            "_id": "624023c8193f2404f8312ccc", // Not same as present in db
            "name": "Name2 Title2",
            "mobile": "CDE393F339"
        },

Schema for user is:

@Schema( )
export class User extends Document {

    @Prop( { required: true } )
    name: string;

    @Prop( { required: true } )
    mobile: string;
}

Another Observation: With the use of _id in dto, all the ids in response are in incremental fashion. eg. cb, cc, cd, ce
& with only id in dto it’s showing ids as original id in db.

Library & Framework Used: NestJs, Fastify, Mongoose, Class-Transformer