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Typically the engines I've seen the exhibit this have a fair amount of carbon deposits on the piston crowns and exhaust valve faces. The intake valve faces also to a lesser extent.
The carbon deposits hold heat and can create hot spots, so that on compression of the fuel air charge, it ignites prior to the spark event.
Most often, using a high quality 93 octane or higher (Shell seems to do well at this) will over a few tanks remove some of the deposits and improve things. Sea Foam in the intake may be of use here as well.
Test the theory with a cheap Harbor Freight borescope camera and 90 degree attachment down the spark plug hole and look at the piston crowns and valve faces.
I suspect oils which are noted having lower consumption also reduce the carbon build up as well.
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