My guess is that this is the point that your oil is about to start laying down varnish...
Paying attention to my L67 on the current OCI, with visible varnish in the valve covers, and using an oil alleged to have superior cleaning properties, the oil was clear on the dipstick for week after week after week, then at around 800-900km, got dark, so I changed the oil (was $2.30/litre, which is crazy cheap for Oz, and the reason for my experiment)...the change was rapid. The removed oil was not see through in any way shape or form in a 3/4" glass bottle. I was hoping that it was transluscent enough to heat and cool, and see if I could make it cloudy at lower temperatures.
Oil both dissolves, and produces varnish. It can hold varnish in solution until it is saturated with varnish, then it lays it down ...somewhere, usually not where it is made in the system.
So if an engine is varnish free, the oil will stay clear for a fair part of an OCI. If the oil is changed before it has made enough varnish to be saturated with the stuff, you will enter a new OCI with an "as new" approach to the oil and engine.
However, once the oil is saturate in service, it has to lay down the new varnish.
Once varnish is there, a new oil change will be dissolving old varnish while the engine is making new...and MAY thus get dark/opaque quite early.
It's only a guess, and my opinion, but I've got a little insight into varnish.