Even taking away age, weight, strength, chi force, no gi etc it can still happen and I'm not talking about the higher grade trying something new or taking it easy or whatever, I'm talking about both guys going as hard as they can at that specific time.
As I am a nerd, please see the chart below;
The red indicates the actual performance level during their roll.
The brown belt is not having one of their better days and is performing around 55% whereas the blue belt is having a great day and is operating around 65%, almost their maximum level.
In conclusion, if you are having a bad day and your partner is having a good day, you are probably going to 'lose' (if there is such a thing as losing but that's for another blog post) so don't be too hard on yourself, accept it, come back again tomorrow and carry on.
Henry Ford, pioneering car manufacturer said "Failure is simply the opportunity to begin again"
That's Jiu Jitsu baby!