I think it depends on what people mean by "explosion". I've seen the guncam video on youtube where a 190 gets attacked and the drop tank is set ablaze. The thing is, it doesn't blow up violently enough to outright destroy the 190.
It sure ignites in a spectacular fashion and i don't know what kind of damage the aircraft sustained as a result, but it's clearly seen having its wings in place and continuing to fly.
In theory, a cannon shell hitting the tank under the right conditions would be able force the creation of a fuel-air mixture/spray that could be ingited by a second shell/tracer or even the residual heat from the first shot: the first shell bursts the tank and the shell's explosion sends fuel flying all around, which could then be ignited as it's now mixed with air.
However, let's also not forget that burning doesn't equal explosion. Many things that can cause explosions under certain circumstances fail to do so under others. In order for an explosion to happen, we also need to have the volatile contents under pressure. So, an empty tank full of vapors but sealed and under some kind of pressure is more probable to explode when hit. A tank that gets holed but doesn't explode on first hit might result in a fuel leak that gives a suitable fuel-air mixture to ignite by subsequent bullets but by that point the pressurization is gone, so the most probable outcome would be the fuel burning but not exploding.
It's just like gunpowder. When sealed in a shell casing and ignited, it's capable of propelling shells at speeds high enough to kill people and shoot down aircraft, but if you open the shell casing, empty the gunpowder on the floor and set it alight it just makes nice sparkles
