Iterative reconstruction has become a popular route for dose reduction in CT scans. One method for assessing the dose reduction of iterative reconstruction is to use a low contrast detectability phantom. The apparent improvement in detectability can be very large on these phantoms, with many studies showing dose reduction in excess of 50%. In this work, we show that much of the advantage of iterative reconstruction in this context can be explained by differences in slice thickness. After adjusting the effective reconstruction kernel by blurring filtered backprojection images to match the shape of the noise power spectrum of iterative reconstruction, we produce thick slices and compare the two reconstruction algorithms. The remaining improvement from iterative reconstruction, at least in scans with relatively uniform statistics in the raw data, is significantly reduced. Hence, the effective slice thickness in iterative reconstruction may be larger than that of filtered backprojection, explaining some of the improvement in image quality.