Also, if the attacker plundered you during your collection, it could be that his plunder updated before your collection. Because a plunder collection is immediate (since there's only 1 thing to collect) and a city collection has a few seconds delay, allowing your to click/sweep a few times and collect everything in 1 update.
Next time, please use the Bug Report Template, found in the top post of the Bug Report section aptly named:
!!!Bug report section - MUST READ ME FIRST!!!
This template is mandatory in order for us to help you to the best of our abilities.
Without the template we usually don't get enough info and can't answer queries properly.
Hence they will be closed and archived after a few days if they're not edited into template form, despite of an answer or not.
- L