There are instances - few and far between - where soils are properly buffered but have a lower pH. This appears to be one of those instances. I do know it's possible, but I don't understand all the chemistry that goes along with it. The soil type itself has an effect on this. I'd take a guess that this soil is either extremely sandy or has a very high clay content/percentage resulting in some somewhat 'abnormal' pH buffering compared to what we are accustomed to. That said, the resulting pH does affect the availability of nutrients to the grasses. I've never had the opportunity to encounter one of these soils so I can't speak too much about it, although I think it would be fascinating to do so.
5.7 is on the low end of what would be considered normal for typical/most grasses. I have encountered grasses performing 'average' at pHs of around 5.2 or so, although there tend to be other indicators present that you don't really want. A high percentage of broomsedge bluestem being one of those indicators.