Question: Multiplying Matrices--editing initial

Maple will multiply two matrices for me perfectly.  

However, when I change the value in one of the cells and try to re-evaluate the product, Maple starts giving me the sum of something in all of my cells.   


Are you simply not allowed to ever change the initial values you set for a matrix?  This doesn't seem to make sense to me.  

I have attached a screen shot to show an example with a random matrix.  

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