Ok, your trying to compare volumetric to surface criteria within the same same system. The use of 1/4" internal lack of fusion implies your comparing radiographic criteria as a comparison.
Lets try the link instead.
http://cstools.asme.org/csconnect/pdf/CommitteeFiles/15243.pdfB31 Case 181-1
Use of Alternative Ultrasonic Examination
Acceptance Criteria in ASME B31.3
You'll note that you can in fact have 1/4" length or less LOF, with the depth to height
Code case 2235-9 and now -10 also shed some light on this.
Historically, RT has had a tighter criteria for lack of fusion and cracks because it's POD (probability of detection) is a) not as efficient as UT for planar type flaws and b) because fracture mechanics were not understood as well as they are today when the criteria was first written.
If you can see it all with RT, there is a good chance there is more to it than meets the eye.
Therefore your going to see a less restrictive criteria for UT that does in fact allow a small amount of planar flaws. (as reflected in the code cases listed above.
On the visual side, the acceptance criteria is well known and accounted for with regards to the specific system in question.
It's almost always a mistake comparing volumetric criteria to surface criteria.
Regards,
Gerald