Perhaps he messed up some inequalities problems to score lower on that section?
Perhaps he had some trouble solving systems of equations?
I am only guessing, as these are some areas that algebra students more often have trouble with.
It is hard to tell from one algebra test much of a pattern about what he is good at.
You probably need to look at how he does in all subjects and what he does for interests outside of school to see if there is any pattern in his way of thinking, what he is good at, future directions of study, etc.
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Drain and plane and grain and blain your brain, and then again,
Propane and butane out of the gas main, your blain shall sustain!