Cucurbits - squashes and their ilk - are notorious about cross pollination. Since they all belong to the same species, Cucurbita pepo, summer squashes like zucchini or crookneck, pumpkins, gourds and many types of winter squash can pollinate each other. This promiscuity will not show up until the next generation and you may never know what the other party involved is as they do not need to be growing in your own garden but only within a reasonable bee flight distance.