Why MAX_PACKET_HEADER_BITS set to be 15 rather than 16 in the version higner than 4.12?

I find a problem that the NET packet doesn’t send success from server to client until I set the MAX_PACKET_HEADER_BITS to 15 which is same as the new version.

So I want to ask that
in the version4.12 the value MAX_PACKET_HEADER_BITS is set to be 16 but 15 in version4.14.
Why do this change in the progress of version upgrade?

UE4.12 NetConnection.h line 28

enum { MAX_PACKET_HEADER_BITS = 16 };

UE4.14 NetConnection.h line 29

enum { MAX_PACKET_HEADER_BITS = 15 };
// = FMath::CeilLogTwo(MAX_PACKETID) +
1

Hoping for your reply~