In other words, only a small percentage of the light passing through the BBO crystal is split into entangled pairs. The vast majority of photons passing through the crystal are not split, and must be removed from the final data set as unwanted noise. Since there is no way for the detectors to measure whether or not a photon had been part of an entangled pair, that decision is made by looking at the timing, and filtering out any photons that are not picked up at the same time as their 'twin' at the other detector. Thus, when a pair of entangled photons is created, but one of the two is blocked by a polarizer and lost, the remaining photon will be filtered out of the data set as if it was one of the many non-entangled photons. When viewed this way, it is not surprising that making changes to the upper path can have an impact to measurements taken on the lower path, as the two measurements are being compared and used to filter the data.