First, let me just point out that resolution is not equal to accuracy.
Second, the BMA220 has a higher bandwidth (lower noise peaks) and selectable bandwidth (flexibility of software/firmware to config for an application.)
I can't find Bosch spec sheets. All links I have found are dead.
My guess is the BMA220 is actually better suited for how it is being used, but of course I don't have access to the iOS source to see how it is interfaced, or the spec sheets to see what integral filters, if any, are present on each.
There are a lot of good articles out there describing what each spec contributes to suitability and accuracy for a particular application.