The chances of a bloon in a round is fairly likely however there are usually only 1 or 2. It appears at around round 100+ (bloons) and round 175+ (blimps). (With a couple of exceptions). There are occasions where blimps can/do have this before they're supposed to (Eg. Round 112) Gravity causes all projectile-based attacks (pretty much the only thing that can avoid this (sorta) is beams) to get pulled towards the bloon/blimp with Gravity. This spans pretty much the entire map and is practically unavoidable; beams are more resistant to this and will only really get bent.