Why starting body fat sets the pace
How aggressively a person can diet without losing muscle depends heavily on how much fat they start with. At higher body fat there is plenty of stored energy to draw on each day, so a larger deficit can be fueled mostly from fat, and a faster cut is reasonable. As body fat drops, the daily fat supply shrinks and a large deficit increasingly pulls from lean tissue instead.
This is why a beginner carrying more fat can often lose quickly with little muscle cost, while a lean individual cannot.
Matching deficit to leanness
A practical principle is to scale the deficit to body fat: more aggressive when there is ample fat to lose, and slower and steadier as you get lean, paired with high protein and resistance training to protect muscle. Very lean individuals generally do best with small deficits and patience.
Specific thresholds vary by person and these are general ranges rather than rules. This is general educational information, not medical or training advice.