Sometimes being so observant can be a curse
For reasons unknown, perhaps even to itself, Mossberg assigns its model numbers to various families of guns, e.g., the 500 series, 590, & 590A1.
It would have made some sense if Mossberg defined the 590A1 series of guns with "clean-out" mag tubes (the mag tube which can be easily taken apart to run a cleaning mop through it, like a Remington 870's); Parkerized or "Mariner" finishes; and metal trigger guards & safeties; regardless of barrel length.
It would have also made sense if the 590 series was very similar except for being a bit less expensive with polymer trigger guards & safeties; and some other finishes like being blued.
Then that would leave the 500 series of having non-clean-out mag tube guns, but they deviated with a couple of those "Tactical", pistol-gripped-breacher-nosed models. I seem to recall there was an old Government/military contract where similar guns were to be marked as a "500" in or near that configuration with that "clean-out" mag tube. Perhaps the truth lies somewhere near that rumor.
My agency has been using a version of the 590A1 for a couple of decades with the models specs slightly changing over the years. What you see one year may not be exactly available the next year. With a large enough quantity special order, Mossberg may be willing to make a gun just the way a customer wants, however.
The best thing would be to have a good idea of what features you'd want and find an existing model which comes close.
Why 7 and 8 round mag capacities on a 20" barrel? Note the mag tube again. One will have an end cap, like a Remington 870; the other will have a captured bolt which threads into the mag tube. I suppose that takes up the space for that extra round.