Compilers typically align struct fields to a boundary to favor execution speed. This may be good for most use cases where there aren't thousands of instances of that struct wasting a lot of memory making it a bad tradeoff.

So C/C++ compilers allow you turn this off either by specifying the data length yourself (bit fields) and/or specifying attribute(__packed__) directive in the case of GCC for example. In this case the packed directive resulted in suboptimal packing so they must have gone with bit fields I suppose to do manual packing.