So far my burpee seeds have been what they claim, but you could be right.
It's a rainy morning so I looked into that cucumber multi pack. Apparently one of the varieties in it is Armenian cucumber, and according to wikipedia it's a Verity of musk melon not a cucumber despite it tastes and looks more like a cucumber. It's also not able to self pollinate so there is a very good chance it will cross pollinate with something else. My bet is that is what you have going on.
The Armenian cucumber, Cucumis melo var. flexuosus, is a type of long, slender fruit which tastes like a cucumber and looks somewhat like a cucumber inside. It is actually a variety of muskmelon, a species closely related to the cucumber. Wikipedia.
Found this pic..