They are assuming owning a total of 2 pets over lifetime, rather than continually having pets – 1 dog who lives 13 years, and 1 cat who lives 11 years.
Pet expenses can be all over the map. If you don’t have major medical expenses, and don’t pay for services like doggy day care, groomers, walkers, trainers, … then expenses can be quite low. I average a few hundred dollars per year on my dog, not including indirect expenses like gas driving to dog beach or dog-friendly hiking areas. However, as you note major medical expenses can occur, such as surgeries costing $7k or substantially more. You could spend <$1k one year, then >$10k the next. Many get insurance to mitigate risk of the latter sudden expense, although average expense is usually lower without insurance.