Comparing acidic strength of phthalic acid isomers

I'd like to compare the acidic strengths of phthalic, isophthalic and terephthalic acids. As I understand, phthalic acid is the most acidic due to steric inhibition of resonance (SIR), but as terephthalic acid has a negative mesomeric effect (−M) operating, it should be more acidic compared to isophthalic acid.


However, the data from Wikipedia suggests that isophthalic acid is more acidic:





| Acid | Structure | $\mathrm{p}K\_\mathrm{a1}$ | $\mathrm{p}K\_\mathrm{a2}$ |
| --- | --- | --- | --- |
| [Phthalic acid](https://en.wikipedia.org/wiki/Phthalic_acid) | [Phthalic acid](https://i.stack.imgur.com/MRRkn.png) | $2.89$ | $5.51$ |
| [Isophthalic acid](https://en.wikipedia.org/wiki/Isophthalic_acid) | [Isophthalic acid](https://i.stack.imgur.com/wVGOm.png) | $\color{red}{3.46}$ | $4.46$ |
| [Terephthalic acid](https://en.wikipedia.org/wiki/Terephthalic_acid) | [enter image description here](https://i.stack.imgur.com/qbSzv.png) | $\color{red}{3.51}$ | $4.82$ |



Also, why is there such tiny difference in their acidic strengths?


