Compress PNG does not pretend a quality slider changes a PNG. Browsers do not expose a PNG quantizer here. Re-encoding still strips extra chunks and can change the size. If you set a target and the PNG is over it, the download becomes a JPEG that the search could fit, and the note says the format changed. Transparency will not survive that JPEG.
The quality slider re-encodes a JPG or WEBP at the percent you pick, and writes a PNG again when the file is a PNG. PNG has no JPEG-style quality step in the browser, so that slider does not change a PNG. Target-size mode binary-searches quality, then shrinks the long edge if even the lowest quality is still too big. A PNG that cannot land under the target is saved as a JPEG, and the note says so. Strip EXIF is on by default. Re-encoding already drops metadata; unchecking the box copies a JPEG EXIF segment back onto a new JPEG when one was present. It cannot put JPEG EXIF into a PNG.
Stay on PNG when you need a clear background. Switch to a target only if the size matters more than the transparency.
Before and after sizes are the real byte counts, and the previews are the pictures the browser produced. A batch downloads as a ZIP. This is not compress PDF. A PDF redraws pages and makes text unselectable. A photo should stay on this page. The file converter changes format without chasing a kilobyte target. A target is a best effort: if the picture cannot get that small without becoming a speck, you get the smallest attempt and a sentence that says the target was missed. The page does not label a 400 KB file as 100 KB.