There were games being written in days where every byte was precious, and the indirection necessary for something like the decorator pattern was just infeasible. There was a lot of direct stat manipulation upon equip & unequip back in the day.
Following that there was the era in which the only way to be competitive in the games space was to write in assembler directly, which would be my guess for how the bug you described happened, since a more sophisticated algorithm should probably have fit just fine by the time we're in the color Mac era but it would have been more painful to write, and nobody has any time for that. Lots of bizarre shortcuts were taken in this era. Some of the posts from Raymond Chen about Windows providing backwards compatibility for games of this era give an idea of the situation, which are related to how the games interacted with the OS rather than with game mechanics but it's a similar process.
Finally, games have this perverse effect going on where if you write down a bunch of effects you want things in your game to have ("armor can add % bonuses to this part of the strike calculation", etc.), it creates a set of boundaries within which the game's systems can function, and the ironic effect of the creation of those boundaries is to immediately bring to mind ways of violating those boundaries and demands from the designers to do so. These violations of the boundaries will tend to be buggy.