It's not likely a bug. Pricing can be like that sometimes. Maybe one is a fully-inclusive insurance rental whereas the other is not? It's hard to comment further without knowing the dates you're talking about.

As for swapping, half the folks here will tell you a swap is no problem and they've done it dozens of times. The other half will tell you that unless something is wrong with the car, they likely will not be amenable to a swap, and even if they are, the won't just let you choose from Ultimate Choice. The reality is probably somewhere in-between. Swaps are at the discretion of the location and probably have a lot to do with luck and who you end up talking with there.