how to calculate correct estimate time for the song ?

Hi ,
I have a project with Up2stream V3
I receive information about the song from UART .
Name and total seconds of now playing song .

example :
ELP:672/394000; wich is equal to 6.56min . but the real time is 6.34
ELP:435/286000; wich is equal to 4.76min . but the real time is 4.46
ELP:726/346000; wich is equal to 5.76min. but the real time is 5.46
ELP:234/445000; wich is equal to 7.41min. but the real time is 7.25

so the error is different every time . And can`t be add to calculation .
The app and the web interface show the correct time , but the time i receive from UART is not correct .

Any assumptions ?