The online casinos do not have their own developers, developing their online slot games. Developers of slot machine games online offer their slot game to all casinos, and it is the online casino that decides which slot machines to implement. That is why most casinos online have games like StarBurst, Book of Death, Legacy of Egypt and many more.
The differences from one online casino to another will often be the casino bonus offers and the slot free spin offers. One casino might offer 100 free spins for StarBurst slot game and another casino offers 200 free spins for StarBurst casino.