Possible vs reasonable
Possible doubt only requires one vector through which something could happen. There could be 99 ways that it would not and just 1 where it is possible, but that one still indeed is possible doubt.
Reasonable doubt is when the odds the doubt is right are sufficiently high that you don't want to take the gamble. Example, you may have 20% doubt that you would fall out of a tree if you climb it, but that 20% may still be high enough for you to think it's unreasonable to take the risk.