GPS with external antenna. Which piece of hardware is the limiting factor for accuracy?

We have a Trimble Geo 7x (cm edition) which can operate with it’s own internal antenna.
It’s spec sheet states that it can theoretically achieve 1.5 cm + 2 ppm VRMS vertical accuracy.

We also have a Trimble Zephyr 3 external antenna.
It’s spec sheet states that it can theoretically achieve 3.5mm + 0.4ppm RMS vertical accuracy.

The internal antenna on the Geo 7x is small and is heavily influenced by site conditions like nearby trees/canopy/buildings.

The Zephyr 3 external antenna is larger, mounted higher up, and has built in shielding to help reduce multipath error etc. So the Zephyr 3 used with the Geo7x improves our reception in sub-optimal site conditions.

Now, if we use the Zephyr 3 external antenna with the Geo 7x in optimal site conditions, the best results (RTK or post-processing) were reported at ~0.5 inches (~1.5cm), but more usually 1"-2". Again, in optimal site conditions.

which piece of hardware is the limiting factor for accuracy when the Zephyr 3 is used with the Geo 7x?
I suspect that it is the Geo 7x that limits the accuracy since the best observed results are in line with the Geo7x specs. And the improved external antenna only helps the Geo7x acquire a signal that then gets reduced in quality by the 7x’s software limitations. To phrase it differently, I suspect that the external antenna lets the Geo7x perform better in sub optimal conditions, but still only to the level at which the Geo7x could on its own in perfect conditions. Effectively, in optimal conditions, the external antenna offers no improvement to the Geo7x because its signal is fed to a less accurate/precise piece of hardware/software