Yes, that condition can happen. I worked in my husband's veterinary business for almost 30 years and have seen this with some patients--usually older ones. It has to do with lots of small blood vessels leaking. It does get better though. Always helps to keep them quiet after surgery.
Just like people, each pet reacts differently.
If you see any pussy discharge from the incision then you need to get him on some antibiotics for infection.
Hope he shows continued progress.
Chris