CST Simulation not matching network analyzer

I created a dipole antenna in CST and built it by hand using componants in my lab. The problem is that when I use the network analyzer to measure the impedance and VSWR, the results are not the same as the simulation.
I can create a matching circuit using ADS or change the measurements of the length/width of the dipole, but I feel that this should not happen.
For example, If I were to make this antenna in bulk quantities, they all should work very similiar to the simulation. I shouldn't have to adjust each dipole individually.
This has happened to a number of different kinds of antennas I began to build to gain experience in the field
Does this usually happen in Antenna Engineering?
What can I be doing wrong and how can I get better at building antennas I simulate in CST?

My guess is that you didnt fabricate the antenna the same way you simulated it. You are most likely measuring the VSWR from your SMA or N connector to the dipole. When you cut a hole in the ground plane, it's important that you fill it with a dielectric, or size the hole to account for air as the medium in that section of your transmission line from the network analyzer to free space.
http://www1.sphere.ne.jp/i-lab/ilab/tool/cx_line_e.htm
I hope this helps.

I wonder how you connected the dipole to the single ended VNA port? You'll need a balun, or at least effective isolation of the cable effect. VNA calibration to the right reference plane is presumed.
Get some ideas from previous threads like this https://www.edaboard.com/thread273612.html

Yes, absolutely! Otherwise the cable become part of the radiating structure (different from simulation!) and results change when you touch the coax shield.