modulation accuracy devm

in most communication system, the modulation accuracy is represented by EVM,but in Bluetooth the Modulation accuracy is represented by DEVM. So my question is:
1. why use DEVM to define the modulation accuracy in Bluetooth?
2. is there some relations between DEVM with EVM?

thank you for your help!

The basic EVM measurement represents the magnitude of the error between an ideal signal and the actual received signal.
The DEVM defined by the Bluetooth, represents the magnitude of the error between two received signals spaced one symbol apart in time. The error is measured after all linear distortions are removed from the received signal, which includes tracking the frequency drift of the carrier.

I did some DEVM simulation with the Bluetooth. It seems that the DEVM is different with the EVM totally. DEVM is more easy to be affacted by the IQ imbalanc than EVM. the EVM's analysis can't be applied to DEVM .