Two conversions that are nothing alike
SVG to PNG is exact. A vector file is a set of drawing instructions, and rendering executes them at whatever size you ask — the output is precisely your artwork, pixel-perfect at 1×, 2× or 4×. Nothing is estimated.
Image to SVG is reconstruction. A raster image has no shapes, only pixels, so tracing has to infer the shapes: it groups the image into flat colour regions and fits curves around them. For artwork that genuinely is flat shapes, that recovers a clean vector.
Knowing which direction is exact and which is inference is most of using this tool well. The first never surprises you; the second rewards the right input and visibly punishes the wrong one.