There are different modifiers for each job:
http://irowiki.org/wiki/Weight#Job_modifiersSN has a job modifier of 0, blacksmith has a job modifier of 1000, that means, the WS has 1000 more weight than the SN. SN has base str of 99, whereas the WS has only 95, so according to base strength, the SN has 120 ( = 30*4; you get +30 weight for each base str) more weight than the WS, 1000 - 120 = 880, which is also the difference between the two max weights in your screenshots.