If you got your seeds from a supermarket squash it’s likely the whole field/tunnel were growing butternut squash so cross-pollinated amongst themselves to produce seeds in your squash that came ‘true’. In a garden setting they are not usually that isolated from other varieties and the squash family will quite happily cross-pollinate amongst themselves to produce seeds of variable hybrid characteristics. So although you have butternut squash fruits now, if you were to sow the seed from them and had different squash growing nearby, you may not get butternut squash from these seeds next year. Could be fun trying though!
(Also, any seed labelled as F1 is a specific cross between two different parents and any seed saved from F1 varieties is unlikely to come ‘true’.)
Hope this isn’t too confusing. Haven’t got much time.
Jan
