Hi Jase,
I wasnt going from the Astronomic link, rather this link
http://astrosurf.org/buil/filters/curves.htm#stronomik CCD Ha filter.
The 13nm seems nicely centered but the 6nm doesnt. I havnt heard anyone else complain about this so maybe the site is incorrect or made a mistake in the measurement.
If it was actually like this, how bad would that be, would it have much effect, it still lets through 90%?
EDIT, I uploaded the spectrum he obtained.
Paul