Just judging from the Boeing website about what they really offer is misleading. You can actually buy the engine with different thrustsettings, however the core engine remains the same (CFM56-7, used for -600 to -900).
For example we bought the 22k version for our -700s (and use a mix of 20k and 22k for our -300s), but we could have bought thrustsettings up to the 26k3 version or down to 18k5. The base engine remains the same, but specific fuel usage and maintenance costs are proportional to the thrustsetting, not to mention the pricetag on the engine. So every airlines has to analyze the usual route structure and performance demands before they decide which version of the engine they buy, exactly what you expected.