Modbus Calibration for pH SEN0604 4 probe senor

Is there any explanation to the calibration process of the SEN0603 - Modbus 4 electrode pH monitor?

We have a single register to right 0x53.

That seems to behave like a single point offset.

For example we read 0x04 (pH register) when in 7.0 ph solution we get 46.

So we write calibration value of 24 and then when we read 0x04 we get 70.

However when we place in ph 4.0 solution and read 0x04 we get a value around 60

So then we rewrite the calibration value at 0x53 to 10 and then when we read 0x04 we get 40 which is correct for the ph 4.0. 

But now if we put it back in 7.0 ph we get a value of 56, requiring the cal to be set back to 24…

Can anyone help with this?