The Wikipedia Heat Capacity Ratio page claims that Argon has a different ratio at -180°C, that would make it the only noble gas that doesn't always hold a ratio of ~1.660, why is that ?
It's because the other heat capacity ratios are for 19°C, while Argon is for -180°C.
For an ideal monatomic gas (the noble gasses being monatomic, but not strictly ideal), $\gamma = \frac 5 3 = 1.6666...$. For a real monatomic gas, these values will vary somewhat with temperature and pressure.
Because Argon is measured at so much lower of a temperature, one could expect that the heat capacity ratio will not be identical to a monatomic gas at room temperature, which is what we see here.