At a guess, because “butterworth: symphonic pieces” is not the album title in the Roon metadata source (it appears to be “Arthur Butterworth: Symphony No. 5”).
Notwithstanding the thought that it would be nice if search worked the way you want (you’re not alone), if you are looking for an album you have named yourself in your local library, you’re probably better off using the filter in ‘My Albums’.
So you surmise that the search does not consider the physical album name in the tag. My first attempt was just “Butterworth” but that didn’t produce the album. So I thought, “hey, just type the album title verbatim.”
I’ve given up trying to understand or predict what search will do - it appears to be a never ending source of angst amongst some Roon users…
In your example, if Roon knew that what you typed was a literal album title from a tag in your local library it might have given a better result; but it doesn’t know, so is giving a wider interpretation that is likely to be a ‘good’ result for more people. It’s not perfect… but they are working on it…