Render Start measures the time it took from the initial request until the browser first displayed any pixels other than the black or white background.
Catchpoint measures Render Start using screen captures taken every 100ms*. As as soon as one of these screen captures contains pixels other than the solid black or white browser background, the time-stamp of that screen capture is recorded as Render Start. Because this is done at 100ms intervals, the Render Start value recorded may be late by up to 99ms.
This is also why the measured Render Start may be higher than metrics for other events which definitely occur after Render Start, such as Document Complete.
*Changes implemented by Google in Chrome v97 and later impact the Render Start metric when using the Chrome monitor. See this article for more information.