
That is more likely to be the answer. Brood bitches appear to be less susceptible to Pyometra to those bitchews that have never been bred from.
I don't know how common it is, as I have now had four bitches having regular seasons into middle age. The youngest is just under four years old, and is expecting her first litter.
A firend had a pet Collie Cross bitch that I just assumed was spayed, and she developed Pyo at 12 years old.
When my freind told me she was off her food and lethargic I didn't even think of Pyometra, until she mentioned excessive drinking! She was quite ill as she had left it several days thinking it was just the warm weather. Came through the spay without problems and lived another two years.
I have heard of bitches having Pyo after a first season.
A very famous show sheltie bitch Ch Myriehew Rosa Bleu, top dog one year had been spayed after her first season sue to this.