The genetics of butternut squash

[maggienetball]

I've had quite a good crop from my butternut squash plants this year. I was especially pleased as I had grown them all from seed saved from one last year's shop bought squash.

However, I have harvested the first one today. It tastes beautiful but has no seeds in it at all. It is fully ripe and completely developed.

I am really confused. It's obviously not a seedless variety as the "mother" produced seeds.

Can anyone explain the genetics of this delicious veg?