You would have to ask a member of the D1.5 and/or D1.1 committees to really get an answer.

Personal speculation, types of stresses involved as far as creating stress risers from the angle of the weld profile coming into the base metals involved. 

The dynamic loading of a bridge is diverse from the static loading of structures (this from a non engineer).  Having said that, there is still a difference also between welder quals and production welds that must be kept straight. 

So, after reading my rambling that can tend to confuse the lines between production and qualification, they are just different codes and especially in the welder quals it may be difficult to know why one has an 1/8" limitation while the other 3/16".

I'll bet Al will know though.

He Is In Control, Have a Great Day,  Brent