Critical itself is technically broken (design-wise I mean) because it both never misses, as disregard defense. It would work well if it had the same chance as Perfect Dodge (that is, up to 10% at the highest setting possible), but we don't, so it needed a quick'n'dirty patch.
Which is why we added Double Damage.
Double Damage will usually do less damage than a critical not only because it respects defense ─ and high level monsters usually have more than 50% defense ─ but also because it can miss, and most important of all: It has higher priority than criticals. (The priority list is: perfect dodge, double damage, critical, perfect hit, standard damage calculation).
The root issue is that criticals as they are currently implemented simply don't work and need to either:
1. have the rates nerfed back to the original specification (up to 10% at highest setting possible, or 15~20% if we assume critical based builds) ─ and this will inevitably require changes to monster database as well, given that in hope to make mobs more resistant to critical hits, they gained luck which while in one hand reduces your chance to hit them with a crit, raises their chance to make a critical hit on you, forcing melee players to invest heavily in luck and ultimately leading to the original issue in first place ─ but also making warriors to worry with an extra stat that every other build usually ignore).
2. have the formula refactored. In Moubootaur Legends, critical hits does +40% damage and respect monster defense (this was also the final decision of RO), always hit and always cause at least 10 points of damage. In The Mana World, this would not work because monster defense was also inflated beyond what would be normally reasonable in a vain effort to rescue the balance and ended up just aggravating the problem.
3. Moubootaur Legends also had refactored Critical Defense formula (but in ML case, it was to rely more on level than on luck because ML devs were all lazy at the time and wanted the monster level to perfectly represent the difficulty, without going through the hassle of assigning stats to the monster ─ we called a senior TMW-Br developer to make char builds back in 2018 and made a master stat table which we just copy-paste to monsters while swapping meaning ─ e.g. if the senior dev had put 40 points in str, we put these points in vit). That unfortunately would also require to review monster database.
So as you can see, any proper fix involves a lot of effort and an inevitable review on Monster Database, which would also require extensive testing. Which is why, in the end of the day, developers decided to make a quick'n'dirty hack fix of forcing critical-boosting items to also have double damage. A clever trick to artificially nerf critical hits without going through the hassle of changing monster stats and wasting months to get these values right.
The only fix I could actually propose is what Hocus once told me: “reduce the critical bonus on Monocle, it is too high”. (Which wasn't done because the Monocle stats is a copy/paste from several other rare items, and as you know, we were avoiding doing massive database changes at the time due to being lazy™ ─ no point fixing Monocle if you don't fix the rest, and no point fixing stuff if you're lazy :b)
TL;DR The reported behavior is correct and intentional. The broken item isn't the Bull Helmet or the Double Damage, but the Monocle and the family of Monocle-like items which we were too lazy to fix at the time.
Hocus, once again, shows he knows his game dev theory book :b
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For the record:
Contributions are welcome.
If you want to fix monocle & cia. item stats, patches are welcome and can be made on https://git.themanaworld.org/legacy ─ and feel free to poke every single developer about how to make a patch.