What's the deal with "Quality value is greater than the maximum length, 5" ?

https://github.com/eclipse-ee4j/jersey/blob/d60da249fdd06a5059472c6d9c1d8a757588e710/core-common/src/main/java/org/glassfish/jersey/message/internal/HttpHeaderReader.java#L191

We have international users whose browsers set the quality score to a length of 6 `q=0.0001`, and so we are seeing these exceptions.

Is "5" an arbitrary length? Why not 6 or 10?