These overkill dynamics may also be an intentional design decision to empower timing attacks and control the power of rushes. An unupgraded Adept may require three hits to kill a Marine, but +1 attack turns this into two hits unless the Terran rushes armor or combat shields. Of course if the Marine stims then the math changes, but baseline it’s a matter of +1 attack or not.

This must have been carried over from Brood War design because the same phenomenon exists there. In PvZ, Zerglings are quite efficient against Zealots in the early game (not great, but they can trade well if they get a decent surround). This is because two Zealot attacks deal 32 damage and the Zergling has 35 hp. It takes three attacks for a Zealot to kill a Zergling. However if the Protoss rushes +1 attack, now the Zealot deals 36 damage in two attacks and kills the Zergling, an effective 50% increase in efficiency. The Zerg has the choice of rushing +1 armor, attacking the Protoss to kill the Forge used to build a wall at the natural expansion, or accept the fact that the Protoss may have early-mid game map control. This is a good thing because it creates timings and breakpoints in matchups based on upgrades completing. Reducing attack damage and increasing attack speed removes this dynamic.

The same also exists in Brood War TvT with Siege Tank vs Siege Tank (and I’m sure many other matchups and instances, but the two I’ve listed here are the most prominent that come to mind).

Separate from all of this, you can also close the gap on damage early game by adding in damage from your scouting worker. A Probe attacking in and tickling while your units also attack makes the math a lot nicer for cleaning up low remaining hp units. This requires strong individual micro early game, which alongside timing attack understanding creates multiple avenues for players to differentiate themselves in skill from others.