Firstly it sounds like you want to do bottom-up worldbuilding, meaning you want to get the basic science straight first. I would recommend worldbuilding YouTuber Artifexian, as he has a series where he starts with constructing the solar system and has currently reached climate mapping. He breaks down the science to a very digestible minimum. For your question, I recommend watching his videos on atmospheres, alien atmospheres and sky and plant-color.
Pressure may vary significantly on "earthlike" planets. Mars has only about 0.006 atm, Earth has 1 atm, Titan has 1.5 atm and Venus 96 atm. Mars and Venus both experienced cataclysmic events in their pasts, the loss of the magnetosphere and a runaway greenhouse effect respectively. Thus they are out of the interesting zone. Looking at Earth and Titan no one could say you went overboard if you vary the pressure on planets you construct by an order of magnitude in both directions (0.1 atm - 10 atm). The upper and lower end might be unsuitable for unsuited humans.
If we found an exoplanet that had the exact same gases in its atmosphere (N, O2, Ar, etc.), albeit at slightly different ratios, would we be surprised by that, or would we expect to see that?
Yes, and No. let me break this down in detail and give you some breathability limits for indefinite survival.
Is somewhat a given since ammonia is common in protoplanetary nebulas and will end up on most planets. As it gets broken apart the nitrogen reacts to N2 molecules, which are very stable and inert. Nitrogen will usually just accumulate over time in the atmosphere.
Free oxygen is only a surprise if you don't expect a biosphere. O2 is so reactive that it will disappear quickly if it doesn't get replenished constantly. O2 may also occur abioticly on worlds where no land is exposed to the atmospere, as nothing can be found to xidise it away with.
Nearly all of the argon in the Earth's atmosphere is radiogenic argon-40, derived from the decay of potassium-40 in the Earth's crust. In the universe, argon-36 is by far the most common argon isotope, as it is the most easily produced by stellar nucleosynthesis in supernovas. - Wikipedia
Argon isn't in the atmosphere by chance but as a result of the alpha decay of potassium-40. This means that a planet with no argon in the atmosphere is either very young (not older than maybe 0.5 byr at most), in a low metallicity system (meaning it would most likely be a planet dominated by water), lost its original atmosphere in a geographically speaking recent event or is an artificial world around a gas giant or black hole or is so small (moon-sized) that it had little radioactive material, to begin with. Usually, you will find argon.
You'll usually find out the maximum possible water content by calculating the water vapor saturation pressure. Calculate your planets global average temperature for it and multiply the maximum saturation with 0.125. That's the average water content of the atmosphere. That said this rule of thumb only holds for planets with large bodies of water. Desert-planets might have significantly less moisture.
Carbon dioxide too is a given and will remain in the atmosphere in moderate concentrations as long there is a functioning carbon cycle, which likely requires plate tectonics. A lot of carbon is stored in the lithosphere as rock. If the planet gets too hot it will be "cooked out", which is the reason for Venuses thick CO2 atmosphere
Is usually generated by biological processes, so if there is life, some methane is likely. Abiotic methane is an option, too but you usally wont find it in significant quantities on Earth-like worlds.
Is a short-lived gas and a sign for extremely strong vulcanism.
A byproduct of having O2 in the atmosphere. Will shield away UV-rays and allow life to colonize the land.
So, no one would be surprised to find very Earth-like atmospheres if the planet in question is very Earth-like. Also finding Mars and Venus-like planets should not surprise anyone, as they show paths along which an Earthlike atmosphere might develope.