How does math.random work here?

From http://wiki.roblox.com/index.php?title=Making_an_Explosion_Course

I don't understand how adding random number from 1-3 can make different parts explode at different times. If I "remove number = math.random(1,3)" and the if condition that checks for 1, my parts would explode at the same time. How does it change the behavior of my "ExplodePart"s from exploding uniformly at the same time to exploding at different times? How does math.random choose which parts explode at random times rather than all parts explode at random times?

function ExplodePart(part)
    number = math.random(1,3)
    if number == 1 then
        explosion = Instance.new("Explosion", game.Workspace)
        explosion.Position = part.Position
    end
end

children = game.Workspace:GetChildren()
while true do
    for _, child in ipairs(children) do
        if child.Name == "ExplosionPart" then
            ExplodePart(child)
        end
    end
    wait(1)
end