I think the "totally different" is a matter of perspective. To me, most of the editions (other than 4E) "felt" much the same. Yes, we switched from backwards math in 3rd edition, how we expressed what our characters were changed, the mechanical bits shifted. On the other hand I just got done playing a PC that was the spiritual successor to my first AD&D dwarven fighter/thief. The difference between the original release and subsequent releases was more pronounced because we lost hobbits, we gained the alignment system and you no longer have to be an elf to be a multiclassed fighter/wizard.
But we're still rolling some oddly shaped polyhedral dice to determine whether we hit, wizards are still casting fireballs, fighters are slapping on armor and whacking orcs. HP still doesn't make a lot of sense, effective bow ranges are still exaggerated and humans are still one of the few races that need light to see in the dark.
As an analogy a Ford Mustang is a completely different car than the original (thank goodness) but it's still identified as the same car because it still targets the same audience and niche while serving basically the same role.