The coil chills the air passing over it. If air moves too slowly (dirty filter, weak blower, blocked ducts) each parcel gets over-chilled → the split reads high. If the refrigerant side is weak (low charge, metering fault) the coil can't chill properly → the split reads low.

Humidity matters: in humid air a chunk of the coil's work goes into condensing moisture instead of dropping the dry-bulb temp, so the expected split is lower. Enter RH and the target band adjusts like the printed target-split charts.

Limits: this is a coarse dry-bulb screen — it points you at airflow vs charge, it does not measure charge. The band assumes roughly correct airflow (~400 CFM/ton). Confirm charge with Superheat & Subcooling.