The difference can be ascribed to the much more uniform culture of the aslan, IMO.
Looking at AM1 Aslan, that appears to be predicting at time of sale. It's 8+ no matter the skill level. At level 4, both dice are predicted.
Bk7 MP, is predicting several days in advance; 3 days per level of skill. Again, the retention number seems to be unmodified.
MT is no guide, either, since, at least with the PDF's, I can't find the tasks.
AM 4 Zhodani matches AM 1
AM7 Hiver matches Bk 7
I will observe that the two 8+ and both die settings are VERY culturally unified: Aslan and Zhodani. In the other two, imperial and hiver, are noted for non-conformity and diverse situations.
Checking TNE says Average Bargaining (the equivalent skill)...