Clock synchronization

Question

Dear Engineer 
 When the advertiser and the scanner enter the connect callback, a timestamp will be printed immediately. The problem now is that the advertiser and the scanner do not seem to enter the connect callback at the same time and there will be a fixed time error of 2800us in the middle. Why is this 
 This is the advertiser's code 
 
 static void connected ( struct bt_conn * conn , uint8_t conn_err ) 
 { 
 
 // update_timestamp(); // 获取当前时间戳 
 if ( conn_err ) { 
 printk ( "Connection failed (err %d ) 
 " , conn_err ); 
 bt_conn_unref ( conn ); 
 return ; 
 } 
 // unsigned int key = irq_lock(); // 禁用中断 
 // 延迟获取时间戳，确保连接完全建立 
 current_timestamp_us = k_ticks_to_us_floor64 ( k_uptime_ticks ()); 
 num ++ ; 
 
 } 
 
 This is the scanner's code 
 
 static struct bt_gatt_discover_params discover_params = { 0 }; 
 
 // 连接回调 
 static void connected ( struct bt_conn * conn , uint8_t conn_err ) 
 { 
 if ( conn_err ) { 
 printk ( "Connection failed (err %d ) 
 " , conn_err ); 
 bt_conn_unref ( conn ); 
 default_conn = NULL ; 
 k_work_submit ( & restart_scan_work ); 
 return ; 
 } 
 
 // unsigned int key = irq_lock(); // 禁用中断 
 local_conn_time_us = k_ticks_to_us_floor64 ( k_uptime_ticks ()); 
 num ++ ; 
 //printk("Connected, local timestamp t2: %llu us
", local_conn_time_us);

if ( ! timestamp_synced ){ 
 if ( conn == default_conn ) { 
 // 启动 GATT 发现 
 discover_params . uuid = & timestamp_uuid . uuid ; 
 discover_params . func = discover_func ; 
 discover_params . start_handle = 0x0001 ; 
 discover_params . end_handle = 0xffff ; 
 discover_params . type = BT_GATT_DISCOVER_CHARACTERISTIC ; 
 
 int err = bt_gatt_discover ( conn , & discover_params ); 
 if ( err ) { 
 printk ( "GATT characteristic discovery failed: %d 
 " , err ); 
 } 
 } 
 } 
 else { 
 // 从第二次连接开始，不再进行 GATT 交互，直接断开连接 
 bt_conn_disconnect ( conn , BT_HCI_ERR_REMOTE_USER_TERM_CONN ); 
 
 } 
 return ; 
 
 }