Well, I don't know why my guidebook (Emerald; Prima) says, quote, "Guillotine: Normal, -- BP, 30 acc, 5 PP. Des: In one blow, you knock out the opponent. Move has know effect if the opponent's level is higher than the casting pokemon, but has a higher chance of succeding if the opponent's lvl is lower than yours."
And my XD (Nintendo Power)guidebook says: "Guillotine/Fissure/Horndrill/Sheer Cold (it says the same thing for all of the moves): --BP, 30 acc, 5 PP. Des: Causes foe to faint; the lower the foes level is than yours, the better ACC becomes; can't hit higher."
Also, When i used Groudon and kyogre's (lvl 100) OHKOs on Mount Battle (XD) never missed on the lower lvl pokemon. From all of this, I make out that it's accuracy raises against lower lvl pokemon. Also, on the higher lvl pokemon, it missed (occasionally). Why would both guidebooks say this if it wasn't true?