Gaps were mostly a sign of sloppy developers and rushed schedules. Compare Tomb Raider 1 on both PC/PSX with Destruction Derby (reflections) and Gran Turismo 1/2 (polyphony).
Yes, there were ways to avoid it (shared vertex calculations), but the effect seen was mostly where there are two separate vertices that are supposed to be at the same position, aren't, because each vertex was positioned with different transformation matrices.