IRC these wide variations are all due to relatively greater fluctuations in the value of the dollar compared to relatively lesser fluctuations in the functional value of gold.

With uneven but too-frequent devaluation events, the US dollar, or indices based on it, certainly does not have enough continuity to make accurate trending across the previous century feasible.

These events are usually reserved for situations when other assets are in extreme flux, exacerbating the difficulty accommodating the discontinuity.

This has been by design.