No that's not a good enough explanation. Candidate B has chosen to exclude that he has two other L2 GCSEs.
I presume a university system would take into account each grade, assigning it some sort of tariff point or. The lack points from those two other GCSEs (presumably L1 qualifications add minimal tarrif anyway), they then have 3 GCSEs less than candidate A on the application. It could well be that candidate A has more tarrif points than A because they have 3 more GCSEs, all at L2 with good grades.
It is still in candidate B's interest to submit all qualifications he has attained. But should he wish to potentially undermine his application he can do. He has earnt his qualifications, he should have the right to decide which ones are more important than the others. Sure, UCAS does nothing unjust in requesting this information... but it should just be a request and not a mandate. Also, this methodology of verification falls flat on it's face when trying to evaluate International candidates who have gone through different education systems - UCAS obviously does not have the power to independently verify whether they have indeed declared all their qualifications. So international applicants might well not upload everything and we would be none the wiser.