Apologies, if I’m wrong about this—it’s something I’ve been meaning to check but haven’t got around to—but one other point that might be relevant factoring in to this calculation (at least in some cases) is empty segments.
@karl_lew will be able to clear things up on this, but just based on my usage, my guess would be that SCV allots an amount of time for each segment. In the case of suttas with a number of empty segments you can see the estimated time calculation drop rapidly as you go through a sutta and skips over empty segments.