If you buy seeds in bulk or at your horticulture store, chances are it will be hybrid seeds. The packets are usually marked F1 (1st generation).
Genetics, in particular traits selection, is very interesting. A rudimentary understanding of genetic inheritance can be by Mendel's Law of Inheritance:
There are a few laws but this is the relevant one. F1 seeds are produced in the second level of the diagramme. All pumpkins produce will have uniform characteristics.
When you plant seeds taken out of an F1 pumpkin, the F2 vines will show parent characteristics that was supressed in F1. The expressed characteristics ratio is 3:1. Bear in mind this is simplification of genetic inheritance because the parent plants may bear a lot more characteristics that maybe passed but overlooked/unexpressed when F1 seeds were produced.
For better undertsanding, these characteristics were found in my (ripening) pumpkins:
The second is actually the smallest of all.