Let's put variables in there:

Month 1 
Month 2 
Total 
Employee A Salary 
[tex]a_1[/tex] 
[tex]a_2[/tex] 
[tex]a_1 + a_2[/tex] 
Employee B Salary 
[tex]b_1[/tex] 
[tex]b_2[/tex] 
[tex]b_1 + b_2[/tex] 
% of above that is Employee A Salary 
[tex]\dfrac{a_1}{a_1+b_1}[/tex] 
[tex]\dfrac{a_2}{a_2+b_2}[/tex] 
[tex]\dfrac{a_1+a_2}{a_1+a_2+b_1+b_2}[/tex] 
Total Benefits (unrelated static number) 
[tex]x_1[/tex] 
[tex]x_2[/tex] 
[tex]x_1+x_2[/tex] 
Employee A Allocated Benefits 
[tex]\dfrac{a_1}{a_1+b_1}\times x_1[/tex] 
[tex]\dfrac{a_2}{a_2+b_2}\times x_2[/tex] 
[tex]\dfrac{a_1+a_2}{a_1+a_2+b_1+b_2}\times (x_1+x_2)[/tex] 
You expect the last row to add up.
But just look at the expressions. They don't!
The reason is essentially the same as the reasons fractions don't add up as a/b + c/d = (a+c)/(b+d): addition doesn't "play well" with division. When you add things, their ratios change. You just have a wrong expectation.
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