Capacitive Voltage Divider and Discrepency in Output Signal
A problem is that LTSpice doesn't like "MHz" for what you think. Instead, LTSpice thinks you meant milliHertz, when you wrote it out that way.

So you need to change that to read "40Meg", instead. That will help you with LTSpice, anyway.



Regarding syntax here, you can use Mathjax (I think it's called that.) Or, at least, the version of it they support here. You use either "\\$" to bracket an in-line equation, or "$$" to bracket a block equation.

See this link: MathJax basic tutorial and quick reference for some good info to try out.



Regarding your scope measurement, I don't know. I think you are saying that you are measuring way more than you expect. Is that correct?

Is this on a solderless breadboard? If so, there is capacitance there (significant enough in your case, as I've seen figures in the several pF range) and it may be affecting \$Z_1\$ far more than \$Z_3\$.