LSM9DS1 threashold interrupt of FIFO mode

[Rin]

Hi, 

 

I am reading datasheet of LSM9DS1, and a little conccerned about the following part:

 

 

Especially about the text: "When a FIFO threshold interrupt occurs, the first data has been overwritten and the
FIFO stops collecting data from the input channels."

Why first data have to be overwritten? It looks more naturel not to be overwritten in FIFO mode.

 

Thank you in advance.