DATA Downloading puzzles


I've made my request on MARS Catalogue and submitted my ERA5 Data retrieving Request successfully.

However, I've found that my outputfile is always smaller than that claimed on "Your Request" Page.(The claim size is 40.6GB, but the actual size is 37.8GB)

Meanwhile, i can't read the grib data.

Even if I add the "'format': 'nc','grid':'0.25/0.25'" keywords into my script, Beacause I want these data in nc format since i can't read with grib format.

And the download size is still smaller than claim size. Is this(some abesent of the data) the reason that i can't read both(nc or grib) format file in MATLAB?
I read gribfile with toolbox and function ncinfo/ncread to get the data in nc file. All failed.

 What to my surprise is  the suffix of the downloading url is grib, still.

Why? should't it has been transfered into nc file before i can retrieve it.