You've already answered your own question. B means that it's got a 533MHz FSB. A (or nothing) means that it's got a 400MHz FSB. (Of course not all resellers are smart enough to comprehend this and will sometimes mislabel.)
As for why the B costs less, people with a motherboard that will only accept a 400MHz FSB CPU are screwed. They can't use the 533MHz FSB CPUs without upgrading their motherboard. (Well, or without OCing their motherboard at the very least.) And so they pay the antiquated hardware tax.
No it's not a real tax, it's just the nature of retail. Either the reseller paid more for the 400MHz FSB versions because the chips went down in price and they refuse to take a loss on them (I paid this much for it and darn it I'm going to make money on it!) or the reseller knows that people who will specifically look for the 400MHz FSB version are in a bind and be willing to pay more than what the CPU is actually worth.
<font color=blue><pre>I'm proud to be an American,
who served my country in the US Air Force,
to protect the rights of my fellow Americans,
to hold protests against others like me.</pre><p></font color=blue>