I have tested on our machine, it works fine, nothing is different.
PF and LF fields can be different. That’s one of the points of why LFs are useful in the first place. Yes, there are naturally differences. There are conversions that must happen between data types — conversions always take time and sometimes can cause loss of precision.
One use of different data types is to have a LF that presents packed or binary numeric fields as zoned digits. This can provide much nicer values for FTP. (But decimal positions and negative values take work.)