Only because we really, really want it to.
We can conjecture that the sticker, not sticky enough to stay on the duct tape (but sticky enough to leave a residue that remained on the "worn surface" of the tape for six months) fell off, then traveled twice as far as the remains were from the curb through underbrush that would discourage a small rodent, re-attached itself securely to a piece of wet, dirty cardboard, and waited patiently to be discovered.
Or we can conjecture that finding a heart sticker of a common size and shape, in an obvious trash dump site in the middle of a huge residential development, a few hundred ft. from an elementary school isn't really all that much of an anomaly.
The discovery of both a "heart shaped residue" and a "similar" heart shaped sticker is enough to drive any sleuthing addict into a true "foam at the mouth" frenzy, but sometimes coincidences do happen. I don't think that this one is all that improbable.