Because of the scaling benefits...
Consider if person A only does 3 units (Ext1). Then they'll only be doing the 2 (which counts 2) and 3 (which counts 1) unit courses. This means (generally) they won't be as proficient as a 4 unit student when it comes to the 3 unit paper because doing 4 unit greatly improves your mathematical ability when it comes to 3 unit.
And you might ask, so what? Well, 3 unit scaling depends on 3 unit candidate's performances in 2 unit... and 4 unit scaling depends on 4 unit candidate's performance in 3 unit (in terms of scaling by UAC for the UAI).
And also, if you work enough, you have 4 very very strong units. And more likely then not, people are going to do (noticably) worse in 1 of their 5 or 4 subjects. So only counting 1 of those units will help with their UAI.
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The Band6 given by BOS is practically a joke because they aren't indicative of the great scaling disparity between Mathematics, Ext1 and Ext2.
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Now consider the same person A did 4 unit (Ext2). Then they'd have 3 units and 4 units counting 2 units each. They'd achieve a higher scaled mark (if they work, of course) in 3 unit and 4 unit, then compared with their performances had they stuck with just 3 units.
The more time you spend on Math, theoretically you improve, and you're performances at lower levels will be better. I mean, if you're doing 3 unit and you compare your 2 unit performances with people who are just doing 2 unit, you'll notice the difference, in general. The same for 4 unit.
Hope that answers your question.