Often, companies use scripts (provided by foundries or by EDA vendors, or developed internally) for automated metal fill generation.
This is much easier and faster than doing manual fill insertion - but, none of these scripts (to the best of my knowledge) take into account parasitics matching problem.
In other words - if you have a precision analog design, then automatically generated metal fill may introduce an asymmetry (mismatch) in parasitic capacitive coupling. That's why, for precision designs, layout engineers oftn opt for manual fill generation.