We tried to review 8th Grade Civics, which might have been a bit too advanced. So let's try 1st Grade Arithmatic:
11-10=1
12-10=2
So now we see that the Republicans will have a two-vote advantage over the Democrats instead of the current one-vote margin. We call that winning.
Harris should have lost her seat to a Republican as a result of the increase in the Republican majority in the Senate-that would have added a Republican member to the JC for your +2.
The democrats want the JC visibility for Harris as she makes her run at the presidency.
The democrats pouted and started holding their breath and the Republicans said "OK, she can keep her seat, we'll just add another seat for us".
The proportion is OK but the democrats were accommodated simply because the Republicans caved.
The increase in the Republican membership on the committee should have come at the expense of the democrats and it didn't.