I found an explanation for this in the blurb from DGM Optics that makes exceptional astronomy filters:
"Specifications
This DGM Optics custom designed filter is well suited for a variety of telescopes and objects. The filter design stresses maximum optical throughput utilizing state-of-the-art optical thin-film designs and materials.
This filter is a “first surface” hard oxide thin-film and is much more durable and long lived than laminated “soft film” designs used by several famous makers. They are also much less prone to the internal reflectance problems that plague laminated filters. Laminated filters can produce a very undesirable “doubling” of stellar images often with one of the doubles being red. Contrary to popular belief, this image doubling is not due to the red sideband component that many nebula filter designs produce, but in many cases is due to laminated glass not being exactly parallel to the protective cover plate after lamination. Many laminated filters display this annoying characteristic.
This filter averages around 1% transmission (optical density of 2) through the rejection region from 540 to 590nm, and average transmission greater than 90% in the passband, and greater than 75% at the 656.3nm H-Alpha line. The design yields a filter with a very symmetrical shape and high optical throughput, for maximum enhancement of nebula with a minimum of loss of stars in the field of view.
The substrate is B-270 and has measured total wavefront values of .25 waves. The combination of first surface optical thin-film technology and a polished, flat substrate enables this filter to be used with high magnification, a real plus for small planetary nebula."
The parent compay of DGM, Omega Optical, makes astronomical filters for other big brands. This manufacturing capacity allows them to experiment and produce niche astronomical filters, including the famous "NPB" ultra high contrast filter.