It's not so strange.

If you think of it this way, a female dog has so many eggs each cycle. So if she mates with more than one male all their sperm is vying to get to an egg. So if sperm from Dog A gets to Egg1 and sperm from Dog B gets to Egg 2 that means puppies with different fathers.

Did that make any sense or did I muddle it up?? LOL

Technically it could happen with humans too, although it would be really rare.