There's some write up here below. That's a summary, going into depth would require a lot of writing.
Lyd & Akustikk | Small rooms and reverberation time
www.lydogakustikk.no
But in short you don't have requirements needed in a small room for reverberation. An acoustic called Theodore Schultz proved that mathetically several decades ago, but knowledge and info doesn't always reach the majority. There's no stochastic sound field or critical distance in a small room, something that's absolutely required for there to be a true reverberation time. Most acousticians are trained in large room acoustics and simply apply the same method to small rooms. But small room acoustics is a very different field that not many are trained in or understand well.
I have been involved in several rooms that were treated in regards to RTx measurements. Sounded very bad despite the RTx looked good and needed to be totally changed. RTx are very misleading when used in a small room, despite that the numbers go down with absorption added. It's like using a wrong mathematical equation. It will change with the numbers you add into the equation, but the result is still wrong.
We can also easily see that when for example using a cumulative spectral decay or if appplying a filtered ETC/impulse. This is also the reason we use two different measuring methods for acoustic products. One is applicable to large venues and one is applicaple to small rooms. Very often, only the first is shown because it looks better, though it's not valid for a small room.
