I thought this was strange too, but I found this information here: http://www.delphibasics.co.uk/RTL.asp?Name=$Align&ExpandCode1=Yes

With $Align On (default), complex data types, such as records have their elements aligned to 2, 4 or 8 byte boundaries, as appropriate to the data type. For example, a Word field would be aligned to a 4 byte boundary.
Which means that A4 and A8 will only differ if the record contains 8 byte members such as Double or Int64.