Most compressors crush skin tones, band your gradients and grey out your shadows. Kleer encodes at full colour resolution, then re-reads every pixel against your original and keeps shrinking only while the two stay identical.
Other tools shrink PNGs by reducing them to 256 colours. That is what bands skin and hardens shadows. Kleer keeps the full 16.7 million.
JPEG normally stores colour at a quarter of the detail. Kleer writes 4:4:4 so lips, lashes and fabric edges stay crisp when you zoom.
Each candidate is decoded again and compared pixel by pixel to your original. Quality only steps down while the difference stays invisible.