Different companies have different business models.
Admiral appears to be geared towards younger drivers in fairly common cars, anything modded or less common tends to get let competitive quotes.
Specialist brokers find the best quote for you and know which insurance companies to try first.
Peter Best tends to deal with limited mile policies and with a 5k limit wanted £850 for me, Admiral quoted £520.
If i had a different car, lived in a different area or had a different job they may be rubbish, that's why it pays to shop around.
What does Admiral have to lose by giving a silly quote for business they do not want?
e.g.
(Age + Occupation + Postcode + Claims)>XYZ car - NCB = Quote
(17yo + Used car salesman + Bradford + 20 fault claims)>Car the insurance company is not familiar with/doesn't like - 50% NCB = Quote
If you take the policy, Win. If you don't, No loss.