Boeing and the "Middle of Market Airliner"

[SeenItAll]

Given we are discussing aircraft designations, can someone explain why some model modifications start at 100 and go up, while other start at a higher number? And while some use 3-digit designations, while others use 1-digit. And why some numbers in a sequence are never used?

727-100, 727-200
737-100, -200, -300, -400, -500, -600, -700, -800, -900, MAX-8, MAX-9
747-100, -200, -300, -400, -8
757-200, -300, (-100 missing)
767-200, -300, -400, (-100 missing)
777-200. -300, X-8, X-9
787-8, -9, -10

300B4, -600
310-200, -300
320-100, -200
318, 319, 321
340-200, 300, -500, -600 (-400 missing)
380-800, -900 (not produced)
350-800, -900, -1000

I have tried to list only major models, but I am sure with some inaccuracy.