>>5981532PNGs have multiple compression levels. All are lossless, so my guess is that they exist so the creator can pick the tradeoff between file size and the time it takes to display one. On 1996 computers.
On modern computers there is no reason to save a PNG at anything but maximum compression. But I've run into a few programs that default to other compression levels. If the user didn't check the compression level, say because they don't know multiple compression levels exist, they will get larger file sizes.
