Consider a double-slit quantum eraser experiment such as the one by Walborn et al (http://arxiv.org/abs/quant-ph/0106078). The overall pattern formed by the signal photons on the screen behind the double slits is the same regardless of whether which-path information is available or destroyed (plot of "N Vs X" looks something like a scaled up Gaussian). In the case that the which-path information is destroyed, the verified explanation is that the net pattern is the sum of two offset interference patterns (one when the idler collapsed to [itex]|+\rangle [/itex] polarization in the Walborn case, and one for [itex]|-\rangle [/itex]). In the case the which path information is accessible, no such interference patterns occur and as far as I know it is a coincidence that the math works out such that the net pattern is the same. Is it really just a coincidence that these patterns are the same? I'm assuming there is an explanation of these experiments which makes this result fall out automatically (something better than just "it has to be that way to preserve causality"), right?