What you get inside the float bowl is concentrated tars that are not removed during refining, concentrated dye ( so we can tell the different grades ) and any other concentrated non volatile agents added to the fuel ( which is not petrol ) and that happens regardless of weather it is leaded or unleaded , ethanol or ethanol free .
Then if the carb bowl was run dry you can still get condensation from the air if the temperatures are low enough & the dir is humid enough ( note , lots of if's )
Now any two metals that are in contact with each other and also wet will allow a galvanic cell to be formed so you will get corrosion in a metal carb bowl where there is steel, zinc & brass
Finally the air we breathe s chockes with viruses , bacteria , pollen & spores of fungi & moulds .
If our eyes were able to see all of them we would not be able to see any of our hand if the outstretched palm placed against our nose , which by the way is why all of those wipes & sprays are mostly just a waste of money .
Now fuel is ORGANIC, it contains the same stuff you are made out of , carbon atoms bonded to mostly hydrogens ( and a host of other elements ) .
To some of the fungal spores & bacteria the fuel or the fuel saturated water or the ethanol phase is FOOD .
So they will grow and multiplay till they consume every drop
Then other fungii , moulds & bacteria will see the previously mentioned fungii, bacteria & moulds as their food, or their waste as food , that is how nature works so they will start to grow & reproduce .
What we see is jelly in a variety of colours