When it comes to infrastructure, companies can only purchase their connectivity from a limited number of companies - so yeah, there are definitely fewer choices, which means a lot of companies use bandwidth from the same place.
Ultimately, it comes down to:
a) what the company does with this bandwidth [do they buy multiple redundant connections, or just buy one and hope it doesn't go down?]
b) what the company does with the hardware connected to the bandwidth [are machines properly configured? how many accounts on one server? etc. etc.]
c) value-added services to complement their bandwidth: security, support, etc. Lotsa companies offer tons of stuff, some don't offer anything.
Adam
http://www.hosts4porn.com
(Where you can read up on lots 'o this stuff)