Frequency response of dual vs single op amp: OPA2890 vs OPA890
Does this make any sense?


Yes it does, from what I've seen they manufactures can use different silicon sometimes for a dual vs a single or quad part. But without actually seeing the die layout there is no way to really know (you could get some nitric acid and microscope -- be careful-- and find out for sure). 


  I would think the same op amp would have the same frequency response,
  or for the single to have better properties than the dual. Of course I
  can just get the dual and use 1, but is this an actual difference
  between op amps or just an error in the datasheet?


The manufactures datasheets should reflect actual testing of the parts, and purchased parts should reflect the test data seen in the datasheets. 

The difference probably a result parasitics, maybe the die wires are shorter in the OPA2890 since they might have put two separate dies in the same package. Or it could be due to more capacitance in the OPA890. Its all in the parasitics of the package or testing, or both. I'd trust the datasheets unless you want to get a part and do the testing on your own, the datasheets should reflect actual testing done by the manufacturers engineers.