Have you checked your actual valve lift and valve clearance? Valve clearance at the high end will increase the compression test results. However, it would be odd that all the cylinders were high. Valve timing can also alter the compression test results. Any chance that you are advanced by one tooth (intake closing sooner than spec) or perhaps there is a flaw in the camshaft? The Volvo service manual sets out a procedure for confirming correct valve timing.

As a note, the suggestions around adjusting ignition timing will do nothing to address dieseling. The ignition is off so ignition timing is moot. I don't think gasoline engines suffer from the true auto ignition that occurs in diesels. Rather, what is occurring is ignition off of a hot surface in the engine. The heating of the charge associated with a high compression ratio along with low octane gas (low octane gas is more reactive) certainly facilitates ignition off of the hot surfaces. The typical source for auto ignition is hot chunks of carbon in the engine. If you carboned up the combustion chamber experimenting with rich mixtures to suppress auto ignition, you may have made your auto ignition problem worse by adding more locations for ignition.

If the dieseling was really bad, the idea of spraying methanol just prior to engine shut down has merit. The methanol will cool the charge plus boost the effective octane rating of the fuel, both of which help to suppress auto ignition. The methanol has the added benefit of helping to keep the combustion chamber clean. The down side is that it could be fussy to set up an automatic system and you definitely do not want to be spraying too much methanol on shut down. Un burned methanol in the combustion chamber would be corrosive.