Disappearing HWI and LWI

The high water inequality (HWI) and low water inequality (LWI) as defined by NOAA are the time in hours after the transit of the moon at the Greenwich Meridian that the high or low tide occurs at the tide station.  It's easier to grasp the reason for computing HWI and LWI if you first think in terms of the local meridian.  Relative to the local meridian, HWI would show (in most places) that high tide follows closely after the transit of the moon, which makes intuitive sense.

So if you go looking for HWI and LWI on the datum of a NOAA tide station, why do some have them and some not?  As far as I understand it, that's due to the tide being diurnal or semidiurnal at that location.  HWI and LWI are not computed for diurnal stations (which can be determined by looking at their "form number" which is a ratio of the primary semidiurnal and diurnal consituents...see The Secret of the Tide).  But for some stations HWI and LWI seems to come and go in the monthly means, for example, Unalaska, AK.  This is a mixed, diurnal station, so I'd guess that HWI and LWI are only computed for months when the tide is more mixed, then diurnal.

I'd be interested to know how that is determined...