According to Cisco’s documentation misses represents “number of times the software performs a translation table lookup and fails to find an entry, and creates one“. So all routers should have some misses in their counters. Let’s look at some 1841 router statistics:
C1841#sh ip nat statistics Total active translations: 3339 (0 static, 3339 dynamic; 3339 extended) Peak translations: 8114, occurred 18:35:17 ago Outside interfaces: FastEthernet0/0 Inside interfaces: FastEthernet0/1 Hits: 28658670 Misses: 0
No misses, only hits that increases every time “the software performs a translation table lookup and finds an entry“. It works same on every ISR and 7200 routers. But still documentation says otherwise.