Because in each view you only know whether there is 0 or at least 1 block in each column. So we can set a lower bound of 35 and an upper bound of 51. The actual number can be anything in between.

Do you know if theres empty space on the inside of those 3 stacks or are they cubes? Not enough information to solve the problem

Think about the two blocks on the right of the side view forming the step in the middle row. What’s behind them? Are you assuming it’s the full truck width of three? How do you know it isn’t a bit of a “false front” and it’s only one deep, or two? What if one is three “deep” and the only is only one? What about the center? Maybe it’s “hollow” on the inside?