I'd noticed the slip ball being off centre but wasn't sure how much that might effect the pitot airflow/readings - the discrepancy is around 15% at this flight configuration. I also realise that the pitot/IAS isn't overly accurate at low speeds but had always thought this was sub-30kt.
Having recently read (and re-read) Shawn Coyle's "Torque Talk" article on the Vertical website about the slip ball, I am as confused as ever about its true function! NB This is not a comment on the author's writing ability but merely my level of comprehension!
If it is parallax error, I assume that would therefore mean that the glass has a common input from one pitot and the 'steam gauge' the other?