I think this topic is very confusing and manufacturers who
make product don't help to explain this. You have to dig
deep into electronics to understand the roots.
This a decent explaination.
http://www.bcae1.com/regunreg.htm
They explain the generic idea behind un-regulated and
regulated. Regulated monitors the output and makes
adjustments to try to hold the output the same. The key
word is 'tries'. The un-regulated may be also regulated,
but without monitoring so it would be a loosely regulated
power supply.
In the link above, they explain stiffly regulated, tightly
regulated, moderately regulated. This is not the electronics
explaination, but rather a basic explaination that helps people understand.
This might give you a deeper understanding of how this
really works. It's the best link I can find right now.
http://www.rantec.com/L2A_Prod/LowVoltage/LVAN/LVAN_HDMA105.pdf
It will explain line and load regulation and there is
a formula. When you design a power supply, this is
the thought process. Car audio amplifiers have DC-DC
converters aka switch mode power supplies [SMPS] and
the article is actually for a certain brand of converter.
The electronics answer is a power supply will have a
certain % Line regulation and a % Load regulation vs.
marketing jingle.