I notice you're using default OGame without plugins. On the default OGame fleet page 2, it's really easy to accidentally change the speed by just overing over the speed bar. And since the speedbar is between target selection and clicking confirm, it's super easy to hover over it by accident.
That is not likely because I would then see it instantly that the fleet is taking that long flight time, which I don't. When sending out resources from one planet to another I seldom tough the speedbar.
When you click "send" and move your mouse, and it lands on the speed bar, the speed will be changed...
And no, you don't notice it, because 1. you dont expect it, 2. its probably hidden behind other, quicker flights
No chance, I checked it even yesterday - I sent out a colony ship. I made sure it was set to 100%, and when looking at the overview it would take 3h and some 10 minutes to get there.
5 hours later I logged in again, and to my stupid surprise that same colonization mission was still ON ROUTE, with still 10 hours + to go. No way I pressed anything else than 100%. And in this instance it also had nothing to do with the maximum amount of planets, because I did not had them. The only 'possible' variable that might have an effect on it, was the upgrading from impulse drive from 5 to 6 (in the mean time), but even this research I never did in previous similar situations.