You are right, those are the usual values for Blu-Rays. 
I found the link
It's a think that is defined by the Blu-ray standard, without the context it seemed like you were implying a general rule.
Cu Selur
Ps.: Okay, after some thinking buffer below < max rate is okay, as long as you don't need a full second of buffering. (which I still find strange)
I found the link
Quote:Maximum allowed buffer, buffer size is in relation with maxrate. Since STD Buffer Delay for Blu-Ray allows maximum 1 second. This mean to divide buffer with maxrate and see if result is less or equal to 1, if not, buffer is need to be reduced to lower value that meet 1 second STD. For BD media, maximum settings is allowed (30000), but if not meet STD Buffer Delay, then need to be reduced to lower value. For DVD media there is no maximum in Blu-Ray specification, but is strongly recommended that use less or equal to 15000, and again need to meet STD delay of 1 second.source: https://forum.doom9.org/showthread.php?p=1399470
It's a think that is defined by the Blu-ray standard, without the context it seemed like you were implying a general rule.
Cu Selur
Ps.: Okay, after some thinking buffer below < max rate is okay, as long as you don't need a full second of buffering. (which I still find strange)