Because TTG is about maximizing viewer trust.

Viewers are much less skeptical of “changes that are going to happen anyway” (#1 below) than they are of “changes that won’t happen unless someone intervenes to make them happen” (#2 below).

1. Some “light”-related aspects (tones and colors) always change on their own each time photographs are reproduced in a different format...

2. . . . but non-“light”-related aspects (forms and shapes) never change on their own when photos are reproduced in a new format.*

*See FAQ #502 for more on the “format” changes.