I'm not sure why anyone would find it UNUSUAL, I mean, BRP (CoC, DQ, etc.) has essentially one mechanic for everything, you roll d% and try to hit a target number. Certain multiples of the target number may produce better/worse results, and combat has some additional rules to handle active defense (parry or dodge) as well as action economy, but its still ESSENTIALLY all the same thing. And games like CoC, which doesn't really focus on combat and elides most of the fancier DQ combat rules, pretty much IS 'single unified mechanic'.
Same with Traveller, roll 2d6+SKILL, 8+ is (usually, at least in early Traveller) a success, period, end of report. Technically there's a parry rule, but the use of primitive weapons is minimal, so it can practically be ignored. Specific situations are often called out in the rules where the 'DC' varies from 8, or there are specific modifiers, and here and there some things like your computer might provide an extra mod. Its an incredibly simple, totally unified, system published in 1977.
And the trend for the past 10-15 years in newer games has been towards this sort of stripped-down single mechanics systems. Outside of D&D the idea of a hodge-podge of dice, tables, different sorts of success criteria, etc. is pretty dead actually.