It doesn't look like you heated the black wire for nearly as long, or nearly as hot. So the flux was not activated, and the solder did not have time to flow.
Timing a hand soldering application, tinning wires, wouldn't seem a very reliable method:
It takes time for the iron to heat up some strands.
Then you have to reliably apply solder to those strands so they begin to tin.
Then you have to add enough solder that it fills in the bundle, without taking heat out and slowing the process.
Then you have to stroke iron and solder along the joint to fully saturate and tin all surfaces, and push off excess flux or dirt that has oozed out of the bundle.
Finally, dragging or shaking off excess solder.
Solderability is better defined by specific test conditions, like heating a cleaned copper plate and analyzing how the solder and flux tins and flows over the plate, and what finish it leaves when it freezes. (I don't know the actual standards, but they are out there, in ANSI/ISO/MIL/etc.)
Tim