I always thought the PSE burst numbers were tied to the (for lack of the proper term) sector number (e.g. D-5, A-4, C-2 etc.) you trigger them while in a playfield. I noticed that I'd get to PSE level 5-7 in a given type then enemies would stop spawning so I'd move, kill a few and see the same PSE type level up again but this time it's lower.
This could all be a coincidence and totally incorrect hypothesizing though.
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