According to Wikipedia : This makes objects smaller than they would appear if the driver viewed them in a flat mirror, or turned and looked at them directly. Since smaller objects appear farther away, the objects seen in the mirror look further away than they really are. If the driver does not consider this, they might make a maneuver (such as a lane change) assuming another vehicle was a safe distance behind, when in fact it was quite a bit closer. The warning is there to remind the driver of this potential problem.
Only a government entity (US Dept of Transportation, regulation 571.111, rearview mirrors) could come up with this.
So, mirrors reflect reality, except for those that are regulated by the government. Great.
Something that faithfully reflects or gives a true picture of something else. Dictionary.com
mirrorearly 13c., from Old French mireor "a reflecting glass," earlier miradoir (11c.), from "look at," from Vulgar Latin. mirer*mirare, from Latin. mirari "to wonder at, admire". Fig. usage is attested from c.1300. The verb meaning "to reflect" is first attested 1820 in Keats's "Lamia." from Online Etymology Dictionary