Some people can more easily drop weigt quickly. A teammat of mine said he dropped 10 pounds in 2 weeks. No drugs, just an idiotic "im only eating salad and oatmeal" mentality. He had tied his 10k PR earlier, and went on to win a major championship after dropping weight.
For one, eat a big dinner, drink some water, and get on the scale before sitting on the toilet in the morning and you can be +2kgs. Weigh yourself two weeks later after a quality ride or race and it isnt so surprising. Second, 5kgs may have just been his annecdotal round number (just speculating, but still).
I'm 6'4" and 154. In a month I'll be 151, healthy and fit at the same time. (not that I stay try to stay that lean for very long). My point is, the ease of adding/dropping weight can vary hugely from person to person.