Z=50 and Xs=0 but SWR is not 1.0:1
Your text says "\$Z=50\$", but your measurement only shows \$\left|Z\right|=50\$. 

If \$Z\$ is not purely real, the reflection won't go to 0, and VSWR won't go to 1. It's possible to design a 1-port device where \$Z\$ is purely imaginary with value \$\pm 50 j\$, and get 100% reflection, meaning VSWR approaches \$\infty\$.

Your actual situation isn't as bad as that, but you don't have a perfectly radiating antenna, so you don't see a perfect VSWR.