First, some background: I will assume the anglophone conventions for Young tableaux in what follows. Given a standard Young tableau $T$ of shape $\lambda$, we can define the cocharge tableau $C(T)$ as follows. Let $n(i)$ be a labeling of the boxes of a $\lambda$-diagram such that $n(i)$ is the box of $T$ containing entry $i$. Let the $n(1)$ box (ie the top-left box) of $C(T)$ be $0$. If entry $n(i)$ of $C(T)$ is $c$, we obtain the next entry as follows: if the $n(i+1)$-th entry of $T$ is below the $n(i)$-th entry, fill box $n(i+1)$ of $C(T)$ with $c+1$. If not, fill it with entry $c$. In other words, every time we have an inversion in $T$, we increase the number we are filling $C(T)$ with. Let $\sum C(T)$ denote the sum of the entries of $C(T)$.
It is clear that the minimum value for $\sum C(T)$ is obtained from the Young tableaux $T$ with first row $1,2,\dots, \lambda_1$, second row $\lambda_1+1, \dots$, and so on. (Filling first left to right, then down.) The maximum value is obtained from the Young tableaux with first column $1,2,\dots, \lambda_1$, and so on. (Filling first top to bottom, then leftwards.) Let the minimum value be $m$ and maximum value be $M$.
My question: I believe that for any $0\leq i\leq M-m$, the number of standard Young tableaux $T$ with $\sum C(T)=m+i$ is exactly equal to the number of standard Young tableaux with $\sum C(T) = M-i$. I've checked this for all partitions of $n$ up to 7 or 8. Does anyone know if there is a reference that has a proof of this, or does anyone have a good explanation for why this should be true?
EDIT: I'll give an example to make clear what I'm asking. Consider, for example, the partition $(3,1,1)$ of 5. There are 6 standard Young tableaux of this shape. Consider the two below $$ T_1 = \begin{pmatrix} 1 & 2 & 3 \\ 4 \\ 5 \end{pmatrix} \hspace{20pt} T_2 = \begin{pmatrix} 1 & 4 & 5 \\ 2 \\ 3 \end{pmatrix} $$ These lead to cocharge tableaux $$ C(T_1) = \begin{pmatrix} 0 & 0 & 0 \\ 1 \\ 2 \end{pmatrix} \hspace{20pt} C(T_2) = \begin{pmatrix} 0 & 2 & 2 \\ 1 \\ 2 \end{pmatrix} $$ with $\sum C(T_1)=3$ and $\sum C(T_2) = 7$. You can show that each of these is the unique tableaux leading to the sum. What I claim, and what is in fact true in this case, is that there is in fact the same number of tableaux with cocharge summing to 4 as there is with cocharge summing to 6. (In this case, this is trivial to verify.)