Data from Ph sensor are like inverted

Hello !!

I have buy the Gravity: Analog pH Sensor / Meter Kit For Arduino // SKU:SEN0161 Brand:DFRobot
I use a arduino mega with the original code from DFROBOT : https://wiki.dfrobot.com/PH_meter_SKU__SEN0161_

I have data result, but it's like inverted.
I use buffer solution ph 4 and i have a result of 7.66
For a solution with ph 8 i have a result of 5,15 .
I precise that i have buy a ph sensor digital pro to be sure of the ph of all the tested solutions.

My connection positive, ground and pin A0 are good, i don't understand...

I think , i have certainly forget anything, but what... maybe someone have the same problem ??

Olivier