I can make a good guess, but I can’t say for certain. With improvements in machining precision by the late 20th century, many parts that formerly required accurate hand fitting with files, etc. to function together correctly no longer did, and if hand fitting was required, it was simple and minimal. In other words, “soft fitting” meant little or no hand fitting of components in the traditional old-school sense was needed. Fewer and less skilled assemblers were needed and payroll cost could be reduced.