Just chances I guess. Often when I look for a specific colony resource, I don't get it. Doing that many searches is just waste of resources. I learned to go with the 2nd or 3rd acceptable choice, and then when searching for next colony, I usually get more lucky.
I've never seen any percentage but it seems to me that higher resource colonies are rarer then lower level resource ones. On top of that it seems different sizes have different behavior - but then again it might just be down to chances. Have not played all those 9 sizes/levels enough to have any reliable data.
Just as I said above, after several unsuccessful searches pick one, conquer it, and hopefully on next you will be more lucky. Or maybe try different size for next search. Really just guessing from my side, but I did seem to be more successful in search for specific when I tried search on another size.
EDIT: Large colony search, got granite on first try. had no Titan Ore which I would be looking for.