Uric acid in the blood can fluctuate a lot.
I have had results between 5.3 and 8 while on a similar diet when I wasn’t taking allopurinol or similar drugs. My average test result was 6.6 which is high enough for crystals to be a problem, especially since I evidently had old crystals from the years when my uric acid always tested above 8.
But your results look different: they are consistently low. A few crystals might occasionally form (for instance after physical exercise) but it shouldn’t matter because any new crystals should dissolve quickly considering the average of your test results. And if you had many old crystals, I think you would see more fluctuations in your test results.
I guess you could still have a few old crystals in various locations. Lowering your uric acid with allopurniol may have destablized them, causing pain. I felt temporary discomfort in many locations when my uric acid fell under 5.
If you are often out in the cold or have cold joints because of poor blood circulation, crystals can form even if your uric acid is relatively low.
There is also evidence that the amount of ferritin in your blood affects uric acid crystallization.