Sometime around 1989 (not sure of the year), Volvo changed over to LH2.4. Your car may be a LH2.4 car.

What this means, relevant to your question, is that the distributor body's rotational position no longer affects ignition timing -- I suspect that you, indeed, have LH2.4 because you wrote that "...The distributor can not be adjusted...", which verifies it to me.

Under LH2.4, your car has a Crankshaft Position Sensor sitting on top of the bell housing. This replaces the sensor (Hall effect?) inside prior year distributors in telling the ECU the position of the crankshaft. You'll also find that your distributor (compared to prior model years) no longer has the weights and springs and stuff that affected ignition timing based on rpm, load (manifold pressure), etc. It's virtually empty inside except for the rotor, the sole remaining purpose of the distributor. It's the ECU that takes all those factors into consideration in ignition timing via software, not mechanical gadgetry. And that's precisely why Volvo redesigned the distributor mount so that you cannot turn it.

Bottom line, the ECU is controlling your ignition timing, and you can no longer do anything about it. Just leave it to the ECU's algorhythm's better judgement.