Question: Matrix values with units not added

I've got a question regarding sum of matrix or arrays.

My problem is that sum of the values of the maxtrices is not calculated, but shown as a sum with the '+' sign. I have tried to recreate the problem with an easier example, but so far I have not managed to do so.

Before posting the whole program, I'd like to post it as a question, if someone has an idea about how I could get an evaluation of the matrix.

Here's a screenshot.

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