Hi Steve,
I would suggest you to get more samples, do the average and then compare. Or else, the change could also be caused by RMS noise.
What's more, ADXL345 needs the turn-on time, you can find this from page 4 and page 5 (item 7) of ADXL345 datasheet. So, after you change the ODR and put the part in measurement mode, please make sure you add enough delay to avoid invalid samples.
Hope these can help.
BTW, what's your applications? Do you need to change the ODR on the fly? As you know, for the applications like pedometer, tap/double tap, fall detector, the algorithm only pay attention to the AC value, so the offset is not that important. Of course, for the applications, like inclinometer, offset need to be calibrated part by part.
BR
Neil