When you specify a certain degree of precision, like 1/16", the values displayed in the Measurement toolbar and by the Dimension tool are rounded to the nearest sixteenth. That’s not the actual dimension of the geometry; it’s for display purposes only. So if you are constructing geometry interactively by drawing with the mouse and reading the numbers in the Measurement toolbar, it’s not only possible that you would see rounding errors such as you are describing, it’s likely. I suspect that there is a tilde (~) displayed with one or more of your dimensions, which means “approximate” or “exact value not divisible by sixteen.”
To avoid that, either increase the precision or, better yet, key in exact values as you work. Also, turn off “length snapping,” which will pull your pointer to the nearest specified length increment.