Code in lists, copied from https://tex.stackexchange.com/a/461589/134574

This is somewhat similar to this question.

The values in the fractions matter because the boxes of the characters have different sizes. The p has a descender below the baseline which the x doesn't, thus when you swap them, the box of the denominator get a little bigger and TeX uses a larger delimiter to make that fit.

You have a few possibilities to work around that (basically the same ones I listed in the linked question):

• You can use a fixed delimiter size (\bigg or \Big, for instance):

   $$\Bigl(\frac{x}{p}\Bigr)\Bigl(\frac{p}{x}\Bigr)$$
   $$\biggl(\frac{x}{p}\biggr)\biggl(\frac{p}{x}\biggr)$$
  

• You can \raise the p so that TeX won't try to use a larger box:

   $$\left(\frac{x}{\raise0.35ex\hbox{$p$}}\right)\left(\frac{p}{x}\right)$$
  

• or you can add an invisible p next to the x so that the delimiter used will be the larger one:

   $$\left(\frac{x}{p}\right)\left(\frac{p}{\vphantom{p}x}\right)$$
  

• Or you can change change TeX's \delimiterfactor (and \delimitershortfall, which I didn't show here) and let TeX adjust the delimiters accordingly:

   $$\delimiterfactor=790
   \left(\frac{x}{p}\right)\left(\frac{p}{x}\right)$$
  
   $$\delimiterfactor=970
   \left(\frac{x}{p}\right)\left(\frac{p}{x}\right)$$
  

Full code:

\documentclass{article}
\begin{document}
% \delimitershortfall=5pt % Default
% \delimiterfactor=901    % Default

$$\left(\frac{x}{p}\right)\left(\frac{p}{x}\right)$$

$$\Bigl(\frac{x}{p}\Bigr)\Bigl(\frac{p}{x}\Bigr)$$
$$\biggl(\frac{x}{p}\biggr)\biggl(\frac{p}{x}\biggr)$$

$$\left(\frac{x}{\raise0.35ex\hbox{$p$}}\right)\left(\frac{p}{x}\right)$$

$$\left(\frac{x}{p}\right)\left(\frac{p}{\vphantom{p}x}\right)$$

$$\delimiterfactor=790
\left(\frac{x}{p}\right)\left(\frac{p}{x}\right)$$

$$\delimiterfactor=970
\left(\frac{x}{p}\right)\left(\frac{p}{x}\right)$$
\end{document}

Output: