It would definitely help if you presented in the table the value that you have on the display and next to the value that the meter sends after BT. I suspect there is more data stored in these values, not just voltage. there is for sure range, switch position and other multimeter settings.
An interesting project. The Manufacturer's User Manual is not specific about the type of value the meter sends, but why do you register for both strings and bytes - and why UTF16 strings? It looks like you are not sharing information that would help us to help you.