Tip for those who are looking at the compression ratios thinking "so what?": look at the run-times. It's a minimum of 3x faster than its contemporaries.
FLAC level 5 encode times should be WAY longer than decode times. FLAC level 0 to FLAC level 5 is a huge step up and encoding should be way longer (like factor of 3). Under no circumstances should FLAC level 5 decode be faster than FLAC level 0 decode. These are basic sanity checks. Something is wrong in the benchmark.
I don't know about FLAC, but from my knowledge of compression, this result seems sensible to me.
Smaller file = less bits to process = faster. FLAC level 5 is expected to give a smaller file than level 0, so it makes sense that decoding it will be faster. Of course, it's possible that some codecs enable more codec features at higher compression levels, which makes decoding slower, so it's not always a given, but higher compression giving faster decode doesn't seem unreasonable.