[Dxbase] Prefix mappings

[Ed K1EP]

Last weekend I did the ARRL SS Phone Contest and the CQ WW CW Contest this weekend.  I imported my log into DxBase2006 and after initializing tables, it told me that I had a new country on 20M SSB.  Since the WW contest was CW and the SS contest was SSB, I was confused.  Certainly I had KL7, KH6, KP2, KP3, etc. already, after having several years of sweeps.  I ran Joe's report that told me new band-mode countries worked in a contest.  It told me that on 20M SSB, I had worked a new country A5, from a QSO with AC0H.  Huh?  Checking the prefix mapping, I saw the following entry:

MAP PFX: AC   Pri PFX: K  OLD1 PFX: A5  Old1 Date: 31-12-1971
MAP PFX: AC0  Pri PFX: K 

Why did it map this AC0 call worked in 2005 to an A5 prefix?  Was the alias list busted?  No, so I checked the call map.  There it was.  AC0H mapped to A5.  So was AC1H, AC5H, AC6H, AC7A, AC7H, and AC8H.  I checked these calls in QRZ and they are valid US calls.  

Why the mapping?