Purchase Appraisal Higher than Rehab Appraisal

In March 2018, my partner (@Mauricio Botero) and I purchased our second property located in W. Germantown, Philadelphia. As we make our way through the BRRRR process, we now have to deal with a situation involving a rather sizable difference between our purchase and refinance appraisal valuations. At purchase, the property's appraised value was set at 210k. However, during the refi the property's appraised value was set at 140k. I know there are many factors that are taken into consideration when performing an appraisal, however a drop in value that size seems a bit drastic - especially post rehab. Is this common? Are there ways to ensure something like this doesn't happen in the future?