Actually, the trick consists in canceling the galleys load, so you get back your own resources.
As such, it's only used for quests (asking to collect 75 K resources, etc.) and doesn't actually steal any resources from the target (nor actually gives any).
If the message says targets have received too much resources already, then it means some players have been actually sending resources to them and didn't cancel the galleys, meaning the resources have actually reached the cities.
Of course, there's a "double galleys trick" consistng in sending a real load that will reach the target in addition to the cancelled one, but it's hard to master, as the galleys need to be sent, reach the target, get back to your city, then be sent again, all during your raid, between the moment you send your troops and the moment they reach it.
Maybe if someone has used this trick, then the city may have reached the maximum resources it can get. This is still quite hard to achieve and generally rare. Some players may also have missed the 50 seconds limit and couldn't recall their galleys, or forgot to recall them...
Finally, the player managing your target may also really have received resources.
And don't tell me they're bots : this doesn't exist. Some cheaters are using scripts to chain create multi accounts, that's why their cities are aligned around the border of the map, as for any newly created cities, but there are still human cheaters behind, and they can sometimes log on manually to use their cities.
After all, they created them for a purpose, and certainly not to advantage YOU, but only themselves. Attacks on these cities probably bother them, so maybe they're exchanging resources between these cities (even using scripts to automate this), just to discourage you, after all...
Anyway, Leonidas has asked for coordinates, so he can track and punish the cheaters. You should report them to Plarium or in private message to Leonidas, so they can investigate, please.