You select PNG, download a smaller file and notice that a smooth background now has bands. Lossless image compression may still describe the final encoding: resizing or color reduction can change the picture before that encoding receives it.

That distinction explains why a PNG export can look different from its source. The file extension does not describe the whole pipeline.

Read the settings as a sequence

Consider a sample diagram with fine text and a smooth colored background. An export might first reduce the dimensions, then replace many colors with a smaller palette, then write a PNG. The final encoding can preserve the reduced image while the earlier steps have already changed its appearance.

Image Compressor states that its PNG output reduces colors when the PNG colors setting is 256 or fewer, including its default setting. Choose Full color to avoid that palette-reduction step. Palette mode also has a tighter output pixel budget: 16 megapixels. Resizing or browser decoding can still affect the picture before PNG encoding. Choose the setting according to what the picture contains, rather than assuming all PNG output is equivalent.

Compress PNG also deserves a settings review before you use it for small text, gradients or graphics that another person will edit. Compare the output with the source at the intended display size. These compressors can return the original when the candidate is not smaller, unless settings require a fresh export. Inspect the reported result rather than assuming every run rewrote the image or removed metadata.

Quality sliders are not interchangeable measurements

A number on a JPEG or WebP quality control is a setting for that encoder. It is not a percentage of original detail remaining, nor a universal scale that can be compared across tools.

Google's WebP compression documentation describes separate lossy and lossless methods. The format name alone therefore does not establish which method a particular export used.

For your diagram, inspect the letters and the smooth background separately. One setting may preserve the text adequately while introducing visible bands in the background. A broad glance at the picture can miss either problem.

Use fresh candidates for a fair comparison

Create each candidate from the same source. Record the settings and file size alongside it. This makes it easier to choose an output deliberately instead of repeatedly modifying a download whose earlier settings you no longer remember.

If the source is an editable design, retain it. A compressed export is usually a delivery copy with a particular purpose, not a replacement for everything you might need from the design later.

Define an acceptance check before choosing the smallest file. For a help screenshot, every important label should be readable. For a gradient graphic, the transitions should remain acceptable at the actual display size. For a file that must preserve pixel values, use an appropriate pixel-preservation workflow and verify those values.

The useful result is a smaller file that meets its stated purpose. A lossless label on one stage cannot answer that question for the entire export.