None. I thought it was a glitch of some sort and wanted to let you know about it.
It's fine. Those fractional readings are just measurement artifacts and excessive decimal precision shown by the overlay. 120.035351872 is effectively 2×60 Hz, and deviations of 0.035 and 0.005 Hz are negligible and won’t affect gameplay. Causes are timers, rounding, VSync/VRR and drivers, not hardware failure. The overlay needlessly shows extra digits, so don’t worry, this is normal monitor/GPU behavior.