To be fair, if I'm reading that right, the difference is "a small light patch on the middle spike"?[0] I can see people missing that easily enough!

[0] Absent contextual cues about height, location, etc., which aren't on the stamp

It means that they used a photo with unclear provenance, or they didn’t check the provenance. For issuing a stamp, you don’t just take a random photo of the statue and eyeball it and say “looks close enough to me”.