Different companies offer different specs because of many reasons, not the least of which is purchasing power. A larger hosting company may be able to receive servers at a discount. Also, some companies oversell resources, providing attractive plans, which they hope that you won't use all of your disk space and bandwidth.

Finally, I'd have to disagree with your blunt statement of "second most important thing". What is the most important? Some people care about uptime, customer support, etc.