The simple answer to this question is because the VU meter does not measure dBu -this is a misconception: the meter is actually reading VU or Volume Units.
Before dBu became the current standard across all audio products, a joint venture between telephone, broadcast and motion picture industries was undertaken in the early 20th century to create a standard unit of measuring sound, this is how the VU meter was created.
In present times the LED or virtual meter which measures in dBu has replaced the need for a mechanical means of measuring sound, but some users prefer them, and they are more frequently used in nostalgic recreations of vintage gear such as the 2A-KT and the 76-KT. A similar meter can also be found on some Behringer amps.
If you are uncertain as to the difference between VU and dBu there is a conversion table attached.