Pathfinder 1E wraith vs spectre

[PnPgamer]

So here I am checking out the stats of wraith and spectre, because I was wanting to give my players 3 different ghost types to encounter.
However! The wraith and spectre are almost identical, but with 2 CR difference! Almost same hp, wraith has HIGHER AC! Is this some sort of oversight or does the 1d6 con drain vs 2 levels drained make a difference? (imo con drain seems more lethal)

 

[nevin]

in high level the 1d6 con is much more wicked, 2 or 3 attacks could kill anyone with a 12 to 18 con and would almost certainly kill someone with a 6 to 10 con, In low levels the 2 level drain is more deadly, 3 attacks a 6 level character is done.