Cut-off frequencies for fractional sample rate adjustment
Don't be confused; you're doing everything correctly here:

$$\begin{align*}
f_{sample,in} &amp;&amp;\overset{\text{interpolate}}{\rightarrow}&amp;&amp; f_{sample,intermediate} &amp;&amp;\overset{\text{decimate}}{\rightarrow}&amp;&amp; f_{sample,out}\\
22 \mathrm{kHz}&amp;&amp;\overset{\uparrow 20}{\rightarrow}&amp;&amp; 440 \mathrm{kHz} &amp;&amp;\overset{\downarrow 11}{\rightarrow}&amp;&amp; f_{sample,out}
\end{align*}$$


To avoid images after interpolating, you will have to suppress all signal above 11kHz, which demands low pass with a cut off of $\frac1{2\cdot20}f_{sample,intermediate}$.
To avoid aliases during decimation, you will have to suppress all signal above 20kHz, which demands a low pass filter with a cut off frequency of $\frac1{2\cdot11}f_{sample,intermediate}$.


Since the filter from 1. implies 2., using one filter is sufficient.

This is a very commonly applied optimization.