Mobile phone firms are a prime example as the type of comapny that might change their ex vat price.
for arguments sake, lets say Mr Tom's Mobile operator has 10m customers on £35 a month contracts, he's actually getting £29.79 before vat.
10,000,000 x 29.79 = £297,900,000
so now to keep customers at £35 a month they change the ex vat to £30.5
10,000,000 x 30.5 = 305,000,000
they've just legally generated an extra £7.1m of revenue