why does this happen and what is the chance of a litter

This happens often with an inexperienced dog who is not fully confident, or not handled by someone experienced/confident themselves or when the bitch just does not hold him in. As long as the bitch is ovulating or is just about to, there is every chance of a litter. If you can mate again a day or two later - even if its another slip mating the chances will be greater of a larger litter. I've had bitches have an apparently copy book mating and long tie and no puppies, and a "blink and you miss it job" and a basket full of puppies!!