Who told you that? The Z-filter algorithm might result in an off center plot if guiding starts with the star off center and a large X-factor is used. This happens because it works in the frequency domain and a constant offset corrresponds to zero frequency so it does not get corrected. But it does not necessarily result in a 1 arc-second shift under all conditions. In fact, it is arguably preferable for the offset to be retained as it results in a better image.
fmozza wrote: During my session, I noticed an issue with calculating RMS. I used PHD2 for guiding, and I use the Z-filter algorithm for DEC control. According to the folks at PHD2, when Z-filter is used, PHD2 shifts the image - RA is centered, but DEC is shifted by 1 arc-second. I'm not sure why, but this will throw off the calculation of RMS due to the DEC shift. Take a look at the attached pictures so you can see what I mean.
But very nice tool!