ADE7880 3 phase 3-wired(Delta mode) How to calculate ITHD,VTHD value

Hello 

I use ADE7880 to analyze total harmonic distortion(THD) in 3 phae 3-wired(Delta connection)

It's OK in ITHD each phase,the value is what I expect

But the value of VTHD is unexpected(each phase), VTHD value is totally different from testing condition

I try to use THD original define to calculate(use XVRMS and FVRMS),but the value is still wrong

Would anyone has this issue too? hope to share how to solve thank you~