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MNE-Python performs all computation in memory using the double-precision 64-bit floating point format. This means that the data is typecasted into float64 format as soon as it is read into memory. The reason for this is that operations such as filtering, preprocessing etc. are more accurate when using the double-precision format. However, for backward compatibility, it writes the fif files in a 32-bit format by default. This is advantageous when saving data to disk as it consumes less space.
However, if the users save intermediate results to disk, they should be aware that this may lead to loss in precision. The reason is that writing to disk is 32-bit by default and then typecasting to 64-bit does not recover the lost precision. In case you would like to retain the 64-bit accuracy, there are two possibilities:
dtype used for saving. However,
this may render the files unreadable in other software packages