I've never played Warhammer Fantasy Battle (WHFB), so I am not that familiar with the full range of suitable Bestiary or its game rules. However, I would suspect Dragons, theoretically, would have a higher PV than the average Greater Daemon type. Thus, I'm not sure how suitable the empirical formula 'Japan's United Gaming Society' was using to convert WHFB monsters to Advanced Heroquest (AHQ) monsters – or how well their categorisation translates.
In the
Advanced Heroquest: Undead Supplement by Graeme Davis, his rough, unfinished, and unofficial notes placed a Vampire Lord at PV 24. AHQ's PV system was possibly meant as a loose guideline rather than a rigid formula, making the idea of a hard cap somewhat arbitrary.
However, quite clearly, when individual monsters reach PV 20 or above, the odds are they will be the equivalent of "bosses" or meant for the 'Character Reference Tables'. Essentially, they will be 'named monsters' or possess incredible strength or abilities – they aren't designed to be "cannon-fodder" replacements.
A question worth asking is: on average, how many generic individual (i.e. single model) monster types would realistically exceed PV 20?
If your Heroes are quite happy to be chewing through a nest of, let's say, several 'Greater Daemons of Tzeentch' (PV 20) in one random Lair room, the odds are that they are overpowered or nearing demigod status anyway.
Therefore, if a 'Keeper of Secrets' was PV 20, and another monster – such as a 'Bloodthirster' wielding a Monster Magic Item that granted it a magical "Fireball" attack – was PV 22, what are the benefits of the PV difference? The impact would be either negligible or diminishing returns. In effect, your heroes would likely be ascending to godhood.
Veteran Heroes in AHQ would typically already possess magical items that mitigate most spell damage or bypass the 'Invulnerable' property. They might also have access to magic items or spells that could instantly destroy or banish a weaker daemon – or severely wound it.
I suspect the reason 'Japan's United Gaming Society' chose PV 20 as the maximum limit was, at least in part, for manageability and simplicity. Their reasoning for this is likely explained – but buried somewhere – in one of their AHQ-related YouTube videos. You'll just have to hope they respond to your enquiries.
Do I personally agree with their self-imposed PV cap of 20? Probably not. However, I can certainly understand why they might have arrived at that decision.
Sarcasm: I am ten times stronger than I pretend to be! Yes-yes! The fools underestimate me, but soon they shall tremble before my might! /Sarcasm