lightroom import dng unexpected changed the width and height of picture

when I import an dng ,  I noticed that the size of dng has been changed for about 10~20 pixels.

I compared the size of dng in lightroom and result from python read dng.  the dng size is 10~20 pixels smaller.

for example,  a dng dimention shown in lightroom is 2048x3072,   when we use rawpy（a python lib） to read the dng, we find the dimentions is 2056x3088.

we want to know how the dng was cliped in lightroom ,  and can we recover the origin dng size.