Never mind. Every beekeeper has feeded sugar to their bees and they know how it goes.
Finski, I can only tell you what I have experienced. I've been open feeding (at 100 yds) my bees periodically since late September with no robbing. As the nighttime temperatures got too low for open feeding, I switched to top feeding the stronger hives. After two days of top feeding, I had robbing on all hives. If you have some other explanation, I'd like hear it. All I can do is tell you what happened.
I have no trouble with top feeding in the spring when there is a stronger flow on. Right now we have only a weak flow. Maybe that makes a difference. Or maybe the bees are more aggressive about robbing as they sense winter approaching.