Just bought it too, so it's mostly conjecture on my behalf, but apparently Sieges can take many turns, so there could be a starbase/Military Base nearby,  high planetary Defences, more ships than your Siege ships can outperform and get to the planet, or any combo of those. That's my understanding how it happens. That might change when I make my first siege ship in game though. The tutorial might explain as well.

If the invasion will take more than 10 turns, you can't invade. You divide the planet defense rating by the siege rating of the fleet to determine the number of turns to invade. I usually invade core worlds with 2 troop transports for speedy results. You might get similar results with 4-6 siege ships.